Combine the milk, sugar, cornstarch, cocoa powder, egg yolks, bittersweet chocolate, vanilla extract, unsalted butter, water, and salt in a saucepan. Cook over medium heat until thickened, stirring constantly.

Remove from heat and let cool before filling the pastry shells.

In a separate bowl, mix the all-purpose flour, cake flour, unsalted butter, brown sugar, and flour until it forms a dough.

Shape the dough into small rounds and bake according to recipe instructions until puffed and golden.