For the Choux Pastry:

1. Preheat the oven: Set to 400°F (200°C). Line a baking sheet with parchment paper.
2. Prepare the dough: In a medium saucepan, bring water and butter to a boil. Once boiling, remove from heat and stir in the flour until the dough forms a smooth ball.
3. Add the eggs: Add eggs one at a time, mixing well after each addition. The dough should be smooth and slightly shiny.
4. Pipe the dough: Transfer the dough to a piping bag fitted with a round tip. Pipe small mounds onto the prepared baking sheet, about 1.5 inches in diameter. If using craquelin, sprinkle it on top.
5. Bake the choux: Bake for 15-20 minutes or until puffed and golden brown. Let them cool completely.

For the Filling: 6. Prepare the mascarpone cream: In a mixing bowl, beat together mascarpone cheese, powdered sugar, and heavy cream until smooth and creamy. Set aside.

For the Soaking: 7. Make the coffee mixture: In a shallow dish, combine brewed coffee and Kahlua.

To Assemble: 8. Prepare the choux: Once the choux are cool, slice them in half horizontally. 9. Soak and fill: Dip the bottom half of each choux into the coffee-Kahlua mixture. Fill with the mascarpone cream mixture and top with the other half of the choux. 10. Finish with cocoa powder: Dust the filled choux with cocoa powder before serving.