diff --git a/src/frontend/servers/y-provider/src/handlers/convertBlocksHandler.ts b/src/frontend/servers/y-provider/src/handlers/convertBlocksHandler.ts
new file mode 100644
index 000000000..cdfeed5f2
--- /dev/null
+++ b/src/frontend/servers/y-provider/src/handlers/convertBlocksHandler.ts
@@ -0,0 +1,72 @@
+import {
+  BlockNoteSchema,
+  PartialBlock,
+  defaultBlockSpecs,
+} from '@blocknote/core';
+import { ServerBlockNoteEditor } from '@blocknote/server-util';
+import { Request, Response } from 'express';
+import * as Y from 'yjs';
+
+import { logger, toBase64 } from '@/utils';
+
+import { CalloutBlock, DividerBlock } from './custom-blocks';
+
+const blockNoteSchema = BlockNoteSchema.create({
+  blockSpecs: {
+    ...defaultBlockSpecs,
+    callout: CalloutBlock,
+    divider: DividerBlock,
+  },
+});
+
+type DocsBlockSchema = typeof blockNoteSchema.blockSchema;
+type DocsInlineContentSchema = typeof blockNoteSchema.inlineContentSchema;
+type DocsStyleSchema = typeof blockNoteSchema.styleSchema;
+
+interface ConversionRequest {
+  blocks: PartialBlock<DocsBlockSchema>[];
+}
+
+interface ConversionResponse {
+  content: string;
+}
+
+interface ErrorResponse {
+  error: string;
+}
+
+export const convertBlocksHandler = (
+  req: Request<
+    object,
+    ConversionResponse | ErrorResponse,
+    ConversionRequest,
+    object
+  >,
+  res: Response<ConversionResponse | ErrorResponse>,
+) => {
+  const blocks = req.body?.blocks;
+  if (!blocks) {
+    res.status(400).json({ error: 'Invalid request: missing content' });
+    return;
+  }
+
+  try {
+    // Create a server editor with custom block schema
+    const editor = ServerBlockNoteEditor.create<
+      DocsBlockSchema,
+      DocsInlineContentSchema,
+      DocsStyleSchema
+    >({
+      schema: blockNoteSchema,
+    });
+
+    // Create a Yjs Document from blocks, and encode it as a base64 string
+    const yDocument = editor.blocksToYDoc(blocks, 'document-store');
+    const content = toBase64(Y.encodeStateAsUpdate(yDocument));
+
+    res.status(200).json({ content });
+  } catch (e) {
+    logger('conversion failed:', e);
+    res.status(500).json({ error: String(e) });
+  }
+};
diff --git a/src/frontend/servers/y-provider/src/handlers/custom-blocks/CalloutBlock.tsx b/src/frontend/servers/y-provider/src/handlers/custom-blocks/CalloutBlock.tsx
new file mode 100644
index 000000000..bbb0850c1
--- /dev/null
+++ b/src/frontend/servers/y-provider/src/handlers/custom-blocks/CalloutBlock.tsx
@@ -0,0 +1,166 @@
+/* eslint-disable react-hooks/rules-of-hooks */
+import { defaultProps, insertOrUpdateBlock } from '@blocknote/core';
+import { BlockTypeSelectItem, createReactBlockSpec } from '@blocknote/react';
+import { TFunction } from 'i18next';
+import React, { useEffect, useState } from 'react';
+import { css } from 'styled-components';
+
+import { Box, BoxButton, Icon } from '@/components';
+
+import { DocsBlockNoteEditor } from '../../types';
+import { EmojiPicker } from '../EmojiPicker';
+
+const calloutCustom = [
+  {
+    name: 'Callout',
+    id: 'callout',
+    emojis: [
+      'bulb',
+      'point_right',
+      'point_up',
+      'ok_hand',
+      'key',
+      'construction',
+      'warning',
+      'fire',
+      'pushpin',
+      'scissors',
+      'question',
+      'no_entry',
+      'no_entry_sign',
+      'alarm_clock',
+      'phone',
+      'rotating_light',
+      'recycle',
+      'white_check_mark',
+      'lock',
+      'paperclip',
+      'book',
+      'speaking_head_in_silhouette',
+      'arrow_right',
+      'loudspeaker',
+      'hammer_and_wrench',
+      'gear',
+    ],
+  },
+];
+
+const calloutCategories = [
+  'callout',
+  'people',
+  'nature',
+  'foods',
+  'activity',
+  'places',
+  'flags',
+  'objects',
+  'symbols',
+];
+
+export const CalloutBlock = createReactBlockSpec(
+  {
+    type: 'callout',
+    propSchema: {
+      textAlignment: defaultProps.textAlignment,
+      backgroundColor: defaultProps.backgroundColor,
+      emoji: { default: '💡' },
+    },
+    content: 'inline',
+  },
+  {
+    render: ({ block, editor, contentRef }) => {
+      const [openEmojiPicker, setOpenEmojiPicker] = useState(false);
+
+      const toggleEmojiPicker = (e: React.MouseEvent) => {
+        e.preventDefault();
+        e.stopPropagation();
+        setOpenEmojiPicker(!openEmojiPicker);
+      };
+
+      const onClickOutside = () => setOpenEmojiPicker(false);
+
+      const onEmojiSelect = ({ native }: { native: string }) => {
+        editor.updateBlock(block, { props: { emoji: native } });
+        setOpenEmojiPicker(false);
+      };
+
+      // Temporary: sets a yellow background color to a callout block when added by
+      // the user, while keeping the colors menu on the drag handler usable for
+      // this custom block.
+      useEffect(() => {
+        if (
+          !block.content.length &&
+          block.props.backgroundColor === 'default'
+        ) {
+          editor.updateBlock(block, { props: { backgroundColor: 'yellow' } });
+        }
+      }, [block, editor]);
+
+      return (
+        <Box
+          $padding="1rem"
+          $gap="0.625rem"
+          style={{
+            flexGrow: 1,
+            flexDirection: 'row',
+          }}
+        >
+          <BoxButton
+            contentEditable={false}
+            onClick={toggleEmojiPicker}
+            $css={css`
+              font-size: 1.125rem;
+              &:hover {
+                background-color: rgba(0, 0, 0, 0.1);
+              }
+            `}
+            $align="center"
+            $height="28px"
+            $width="28px"
+            $radius="4px"
+          >
+            {block.props.emoji}
+          </BoxButton>
+
+          {openEmojiPicker && (
+            <EmojiPicker
+              categories={calloutCategories}
+              custom={calloutCustom}
+              onClickOutside={onClickOutside}
+              onEmojiSelect={onEmojiSelect}
+            />
+          )}
+          <Box as="p" className="inline-content" ref={contentRef} />
+        </Box>
+      );
+    },
+  },
+);
+
+export const getCalloutReactSlashMenuItems = (
+  editor: DocsBlockNoteEditor,
+  t: TFunction<'translation', undefined>,
+  group: string,
+) => [
+  {
+    title: t('Callout'),
+    onItemClick: () => {
+      insertOrUpdateBlock(editor, {
+        type: 'callout',
+      });
+    },
+    aliases: ['callout', 'encadré', 'hervorhebung', 'benadrukken'],
+    group,
+    icon: <Icon iconName="lightbulb" $size="18px" />,
+    subtext: t('Add a callout block'),
+  },
+];
+
+export const getCalloutFormattingToolbarItems = (
+  t: TFunction<'translation', undefined>,
+): BlockTypeSelectItem => ({
+  name: t('Callout'),
+  type: 'callout',
+  icon: () => <Icon iconName="lightbulb" $size="16px" />,
+  isSelected: (block) => block.type === 'callout',
+});
diff --git a/src/frontend/servers/y-provider/src/handlers/custom-blocks/DividerBlock.tsx b/src/frontend/servers/y-provider/src/handlers/custom-blocks/DividerBlock.tsx
new file mode 100644
index 000000000..9d402a823
--- /dev/null
+++ b/src/frontend/servers/y-provider/src/handlers/custom-blocks/DividerBlock.tsx
@@ -0,0 +1,51 @@
+import { insertOrUpdateBlock } from '@blocknote/core';
+import { createReactBlockSpec } from '@blocknote/react';
+import { TFunction } from 'i18next';
+
+import { Box, Icon } from '@/components';
+import { useCunninghamTheme } from '@/cunningham';
+
+import { DocsBlockNoteEditor } from '../../types';
+
+export const DividerBlock = createReactBlockSpec(
+  {
+    type: 'divider',
+    propSchema: {},
+    content: 'none',
+  },
+  {
+    render: () => {
+      // eslint-disable-next-line react-hooks/rules-of-hooks
+      const { colorsTokens } = useCunninghamTheme();
+
+      return (
+        <Box
+          as="hr"
+          $width="100%"
+          $background={colorsTokens['greyscale-300']}
+          $margin="1rem 0"
+          $css={`border: 1px solid ${colorsTokens['greyscale-300']};`}
+        />
+      );
+    },
+  },
+);
+
+export const getDividerReactSlashMenuItems = (
+  editor: DocsBlockNoteEditor,
+  t: TFunction<'translation', undefined>,
+  group: string,
+) => [
+  {
+    title: t('Divider'),
+    onItemClick: () => {
+      insertOrUpdateBlock(editor, {
+        type: 'divider',
+      });
+    },
+    aliases: ['divider', 'hr', 'horizontal rule', 'line', 'separator'],
+    group,
+    icon: <Icon iconName="remove" $size="18px" />,
+    subtext: t('Add a horizontal line'),
+  },
+];
diff --git a/src/frontend/servers/y-provider/src/handlers/custom-blocks/index.ts b/src/frontend/servers/y-provider/src/handlers/custom-blocks/index.ts
new file mode 100644
index 000000000..34a8c459c
--- /dev/null
+++ b/src/frontend/servers/y-provider/src/handlers/custom-blocks/index.ts
@@ -0,0 +1,2 @@
+export * from './CalloutBlock';
+export * from './DividerBlock';