Prepare Chocolate Pastry Cream. In a medium bowl, whisk together sugar, cocoa powder, and cornstarch. Add yolks and mix until smooth. Heat milk and sugar until simmering, then whisk into egg mixture. Return to heat, stir in chocolate, and whisk until thickened. Strain, mix in butter and vanilla, refrigerate for at least 3 hours.

Make Chocolate Craquelin. Combine butter and brown sugar in a bowl, beat until fluffy. Mix in flour, cocoa powder, and salt until combined. Roll between parchment to 1/8-inch thickness, freeze for 30 minutes, cut into circles.

Prepare Cream Puff Shells. Boil water, milk, butter, sugar, and salt in a saucepan. Remove from heat, stir in flour and cake flour until a dough forms. Cool slightly, then add eggs one at a time until smooth.

Pipe and Bake Cream Puffs. Preheat oven to 400°F. Pipe dough onto a baking sheet, place a craquelin disc on top. Bake for 20-25 minutes until golden brown.

Fill Cream Puffs. Cool puffs on a rack, fill a piping bag with chilled pastry cream. Fill each puff gently from the bottom.

Chill and Serve. Chill filled puffs in the refrigerator for 30 minutes. Serve chilled as a dessert, or alongside tea for a perfect treat.