In a saucepan, combine whole milk, granulated sugar, cornstarch, cocoa powder, and salt. Whisk over medium heat until thickened.

Remove from heat and stir in egg yolks, bittersweet chocolate, vanilla extract, and unsalted butter until smooth. Set aside to cool.

In another saucepan, bring water and 4 tablespoons of unsalted butter to a boil. Add all-purpose flour and cake flour, stirring until a dough forms.

Transfer dough to a mixing bowl, and let cool slightly before mixing in the egg yolks one at a time until fully incorporated.

Pipe the dough onto a baking sheet and top with brown sugar and additional flour mixture.

Bake at 375°F (190°C) until puffed and golden brown, approximately 25 minutes.

Remove from oven and let cool before filling with the chocolate cream.