In a medium saucepan, whisk together the granulated sugar, Dutch-process cocoa powder, and cornstarch. Slowly add in the egg yolks until fully combined. Heat the whole milk and the remaining sugar until it simmers. Gradually pour hot mixture into egg mixture while whisking to avoid scrambling. Return to heat and whisk until thickened, about 5 minutes. Strain into a bowl, stir in chocolate, butter, and vanilla extract, cover with plastic wrap, and refrigerate for at least 3 hours.

Combine unsalted butter, brown sugar, flour, cocoa powder, and salt in a bowl. Mix until uniform dough forms. Roll out between parchment paper to 1/8 inch thick. Freeze for 30 minutes. Cut out discs to place atop cream puffs.

In a saucepan, combine water, milk, unsalted butter, granulated sugar, and kosher salt, bringing it to a rapid boil. Reduce heat and stir in all-purpose and cake flour until the dough forms a ball. Cook for an additional minute then cool slightly. Incorporate eggs one at a time until glossy.

Preheat oven to 400°F (200°C). Line a baking sheet with parchment. Pipe small mounds of dough, leaving space. Place frozen craquelin discs on top and bake for 20-25 minutes until golden brown. Cool completely on a wire rack before filling.