Make the Pastry Cream: Heat the milk in a saucepan until it begins to boil.

In a separate bowl, whisk together egg yolks, sugar, and cornstarch until smooth.

Gradually add the hot milk to the egg mixture, stirring constantly to prevent curdling.

Pour the mixture back into the saucepan and cook over medium heat, stirring continuously until the cream thickens.

Remove from heat, stir in butter, cover with plastic wrap (touching the surface to prevent a skin from forming), and cool completely in the fridge.

Prepare the Brioche Dough: Dissolve the yeast in lukewarm milk and let it sit for 3 minutes.

In a mixing bowl, combine flour, salt, eggs, and the activated yeast mixture.

Start kneading, then add the vanilla butter gradually, continuing to knead until the dough is smooth and comes away from the bowl's sides. If the dough is too sticky, add a little extra flour.

Oil your hands, form the dough into a ball, and let it rest for 1 hour, or until doubled in size.

Tip: To speed up the fermentation, place the dough in the oven (turned off) with a bowl of hot water.

Shape and Fill the Brioche: Punch down the risen dough and roll it into a rectangle, approximately 28 x 45 cm (11 x 18 inches).

Gently stir the chilled pastry cream to relax it, then spread it over half of the dough.

Sprinkle chocolate chips generously over the cream, then fold the other half of the dough over the filling.

Cut the filled dough into strips about 5 cm (2 inches) wide.

Rest, Glaze, and Bake: Cover the brioche strips and let them rest for another 30 minutes.

Preheat the oven to 180°C (350°F).

Brush the brioche with the egg yolk and milk glaze.

Bake for 12 to 15 minutes or until golden brown.

Cool and Serve: Once out of the oven, cover the brioche with a cloth to maintain softness. Serve warm or at room temperature.