Preheat the oven to 400°F (200°C).
In a medium saucepan, combine water and butter; bring to a boil.
Add flour all at once, stirring vigorously until the dough forms a ball.
Remove from heat and let it cool for 5 minutes.
Add eggs one at a time, mixing well after each addition.
Transfer the dough to a piping bag fitted with a large round tip.
Pipe 2-inch circles onto parchment-lined baking sheets, leaving space between each one.
Bake for 25-30 minutes, until golden brown and puffed.
Let the cream puffs cool completely on a wire rack.
In a bowl, whip the heavy cream with powdered sugar and vanilla until soft peaks form.
Fill each cooled puff with whipped cream using a piping bag.
Make the ganache by heating chocolate and powdered sugar until smooth.
Drizzle or dip each filled puff in the ganache.
Assemble the cream puff cake by stacking layers of cream puffs and securing with extra ganache as needed.
Chill the cake before serving to set the ganache.