Preheat the oven: Preheat your oven to 400°F (200°C). Line a baking sheet with parchment paper or a silicone baking mat.

1. Prepare the choux dough: In a medium saucepan, combine the water, milk, butter, sugar, and salt. Heat over medium heat, stirring occasionally, until the butter is completely melted and the mixture is just beginning to simmer.
2. Add the flour and cocoa powder: Quickly add the flour and cocoa powder all at once, stirring vigorously with a wooden spoon. Continue stirring until the mixture pulls away from the sides of the pan and forms a smooth dough ball. This will take about 1-2 minutes.
3. Add the eggs: Remove the saucepan from heat and let the dough cool for a few minutes. Add the eggs, one at a time, mixing thoroughly after each addition. The dough should be smooth, glossy, and slightly sticky.
4. Pipe the dough: Transfer the dough to a piping bag fitted with a large round tip. Pipe small rounds of dough (about 1 1/2 inches in diameter) onto the prepared baking sheet. Leave about 2 inches between each round to allow for expansion.
5. Bake the choux buns: Bake for 20-25 minutes, or until the buns are puffed up and golden brown. Do not open the oven door during the first 15 minutes to ensure they puff up properly.
6. Cool the buns: Once baked, turn off the oven and leave the buns in for 5 more minutes with the door ajar to allow them to set. Remove the buns from the oven and transfer them to a wire rack to cool completely.
7. Make the chocolate ganache: In a small saucepan, heat the heavy cream over medium heat until it just begins to simmer. Remove from heat and add the chopped chocolate. Let it sit for 2-3 minutes, then stir until the chocolate is fully melted and the ganache is smooth.
8. Sweeten and flavor: If desired, add powdered sugar to sweeten and vanilla extract for added flavor. Stir well to combine.
9. Cool the filling: Let the chocolate ganache cool to room temperature, then refrigerate it for 20-30 minutes to thicken.
10. Whip the ganache: Once the ganache has cooled and thickened, use a hand mixer to beat it until it becomes light and fluffy, resembling whipped cream.
11. Fill the buns: Use a sharp knife to carefully cut a small slit in the side or top of each choux pastry. Pipe the whipped chocolate ganache into each bun, filling them completely.
12. Serve: Once filled, you can dust the tops with powdered sugar or drizzle additional melted chocolate on top for a decorative touch.