In a medium bowl, whisk together the granulated sugar, cornstarch, and Dutch-process cocoa powder. Gradually whisk in the egg yolks until smooth.

In a saucepan, heat the milk with the remaining sugar over medium heat until it simmers. Slowly add the hot milk to the egg mixture while whisking continuously to prevent scrambling.

Return the mixture to the saucepan, stir in the chopped bittersweet chocolate, and cook until thick and bubbling, around 3-5 minutes.

Strain the mixture to remove lumps, then stir in the unsalted butter and vanilla extract. Cover and chill for at least 3 hours.

In a separate medium bowl, cream the unsalted butter and brown sugar together until light and fluffy, about 2-3 minutes. Mix in the flour, cocoa powder, and a pinch of salt until fully combined.

Roll the mixture out between two sheets of parchment paper to a thickness of about 1/8-inch. Freeze the rolled craquelin for at least 30 minutes until firm.

In a saucepan, combine the milk, water, unsalted butter, granulated sugar, and kosher salt. Bring the mixture to a boil over medium heat.

Once boiling, remove from heat and stir in the all-purpose flour and cake flour all at once, mixing vigorously until a dough forms and pulls away from the sides of the pan, about 1-2 minutes.

Allow the dough to cool slightly for 5-10 minutes before adding the eggs one at a time, mixing well after each addition until the pastry is smooth and glossy.

Preheat your oven to 400°F (200°C). Line a baking sheet with parchment paper. Using a piping bag fitted with a large round tip, pipe the choux pastry into mounds about 1.5 inches wide, spaced apart on the baking sheet.

Remove the frozen craquelin from the freezer and cut into circles to place atop each puff. Bake in the preheated oven for 20-25 minutes, or until puffed and golden brown.

Let the cream puffs cool completely on a wire rack before filling them with the chocolate pastry cream.