Start with the cream:

• Soak the gelatin sheets in cold water.
• Chop the chocolate finely.
• Heat the cream to boiling point and stir the gelatin into the hot cream.
• Pour the cream over the chocolate and stir until melted together.
• Set aside in the fridge while you make the choux pastry.
•  
• Preheat the oven to 200 degrees Celsius (392 degrees Fahrenheit).
• Bring butter and water to a boil in a saucepan.
• Add flour, cocoa, sugar, and salt, and stir well until the dough releases from the sides of the pan. Let the dough cool for about 10 minutes. Then beat in the eggs one at a time - the dough should be fully combined before adding the next egg.
• Transfer the dough to a piping bag fitted with a large round nozzle, and pipe it into round mounds about 5 cm in diameter on a baking sheet lined with parchment paper.
• Bake the choux pastries in the oven for about 20 minutes. Let them cool.
• Take your cream out from the fridge. It should be completely cold, check the bottom of the bowl.
• Whisk the cream with a hand mixer until you get a fluffy mousse. Cut the choux pastries in half, fill a piping bag fitted with a large star nozzle with the cream, and pipe it onto each bottom half of the choux pastries.
• Place the top halves on and decorate with glaze and optionally pistachios.