Prepare the Chocolate Custard:

• Whisk the egg yolks and sugar until pale.
• Mix in the flour until smooth.
• Heat the milk and vanilla seeds until nearly boiling.
• Slowly whisk half the hot milk into the egg mixture, then return to the pan.
• Bring to a boil, whisking constantly for 1 minute.
• Remove from heat and stir in the chocolate until melted.
• Transfer to a bowl, cover with baking paper to prevent a skin, and cool.

Make the Brioche Dough:

• In a stand mixer with a dough hook, combine milk, egg, honey, and olive oil.
• Add flour, then place yeast on one side and salt on the other.
• Scatter the butter over the top.
• Mix on low for 4 minutes, then on medium for 10-12 minutes, until smooth.

First Rise:

• Lightly flour a clean bowl.
• Shape the dough into a ball, cover, and rest for 45 minutes.

Second Rise:

• Turn out, knead briefly, reshape, and rest for 30 minutes.

Shape the Brioche:

• Roll out dough to a 40cm x 30cm rectangle.
• Spread cooled custard over the surface and sprinkle with sultanas (if using).
• Roll up lengthwise like a Swiss roll and cut in half.
• Cut each roll lengthwise to expose layers, then twist into plaits.
• Place on lined baking trays, cover, and prove for 45-60 minutes.

Bake the Brioche:

• Preheat oven to 190°C fan/gas 6½.
• Brush loaves with beaten egg glaze.
• Reduce temperature to 180°C fan/gas 6 and bake for 20-25 minutes until golden.

Make the Rum Glaze:

• Heat sugar, rum, and water until it becomes a thick syrup (~8 minutes).
• While warm, brush generously over the brioc