Prepare the Brioche Dough:

• In a mixing bowl, combine the flour, sugar, yeast, and salt.
• In a separate bowl, whisk together the eggs, milk, and vanilla extract.
• Gradually add the wet ingredients to the dry ingredients and mix until the dough comes together.
• Knead the dough for about 10 minutes, either by hand or with a stand mixer, until it becomes smooth and elastic.
• Add the softened butter, a little at a time, kneading it into the dough until fully incorporated. Continue kneading for another 5-7 minutes until the dough is soft and slightly sticky.
• Cover the dough with a damp cloth and let it rise in a warm place for 1-2 hours, or until doubled in size.

2. Make the Vanilla Custard:

• In a saucepan, combine the egg yolks, sugar, cornstarch, and a pinch of salt. Whisk until smooth.
• Gradually pour in the milk while whisking continuously.
• Place the saucepan over medium heat and cook, whisking constantly, until the custard thickens, about 5-7 minutes.
• Once thickened, remove from heat and stir in the vanilla extract.
• Pour the custard into a bowl, cover with plastic wrap (press it directly onto the surface of the custard to prevent a skin from forming), and let it cool.

3. Shape the Brioche Rolls:

• Once the dough has risen, punch it down and divide it into 12 equal pieces.
• Roll each piece into a ball and create an indentation in the center of each ball.
• Spoon a small amount of vanilla custard into the center of each dough ball, then sprinkle with chocolate chips and a small dusting of cocoa powder (if using).
• Carefully pinch the dough around the filling to seal the custard inside.
• Place the filled dough balls into a greased baking pan or lined with parchment paper, ensuring they are close but not touching.

4. Let the Dough Rise:

• Cover the pan with a clean kitchen towel and let the dough rise for another 30-45 minutes until puffed and doubled in size.

5. Bake the Brioches:

• Preheat the oven to 375°F (190°C).
• In a small bowl, whisk together the egg and milk for the egg wash. Brush the tops of the brioches with the egg wash for a golden, glossy finish.
• Bake the brioches in the preheated oven for 18-22 minutes, or until golden brown on top and a toothpick inserted into the center comes out clean.
• Let the brioches cool slightly before serving.

6. Serve and Enjoy:

• Serve warm or at room temperature. Enjoy the soft, buttery brioche with a burst of chocolate chips and creamy vanilla custard inside.