Heat the milk in a saucepan, then whisk in the sugar, cornstarch, cocoa powder, and salt until smooth.

In a separate bowl, mix the egg yolks and temper them with the hot milk mixture.

Return the mixture to the saucepan and cook until thickened, then remove from heat and add the bittersweet chocolate, vanilla extract, and butter.

Cool the chocolate cream filling in the refrigerator.

In another saucepan, heat water and butter together until boiling.

Stir in both flours at once and mix until the dough forms a ball.

Remove from heat and let cool slightly before mixing in the eggs, one at a time, until fully incorporated.

Pipe dough onto a baking sheet and sprinkle with craquelin mixture.

Bake until puffed and golden brown, then let cool before filling with the chocolate cream.