Heat the milk in a saucepan over medium heat until it just steams, do not boil.

Whisk egg yolks, sugar, and cornstarch in a bowl until pale and smooth.

Pour half the hot milk into the yolk mixture while whisking, then pour it all back into the saucepan.

Cook over medium-low heat, whisking constantly, for 3-4 minutes until thick enough to coat the back of a spoon.

Remove from heat, stir in chopped chocolate, butter, and vanilla until smooth.

Press plastic wrap directly onto the surface and chill for at least 2 hours until firm.

Warm the milk to about 105 F / 40 C and stir in the yeast, let sit 5 minutes until foamy.

Combine flour, sugar, and salt in a stand mixer bowl.

Add the yeast mixture, egg, egg yolk, and vanilla, mix on low until a shaggy dough forms.

Add softened butter in two additions, kneading 8-10 minutes until the dough is smooth and stretches without tearing.

Cover and let rise in a warm spot for 60 minutes, until doubled in size.

Punch down the dough and divide into 12 equal pieces, about 65 g each.

Flatten each piece into a disc, spoon 1 tablespoon of chilled chocolate cream into the center.

Gather the edges up and pinch tightly to seal, then place seam-side down on a lined baking sheet.

Cover loosely and let proof 30-40 minutes until visibly puffy.

Brush tops with egg wash and bake at 180 C / 350 F for 18-20 minutes until golden and the internal dough temperature reads about 190 F / 88 C.

Cool on the tray for 10 minutes before serving so the filling sets slightly.