Preheat oven to 400º F.
In a small saucepan melt butter and water together until it comes to a rolling boil.
Stir in the flour, reducing heat to low. Stir vigorously, over low heat for 1 minute, or until the dough forms a ball. Remove from heat.
Using a wooden spoon, beat in all of the eggs at once. Continue beating until it forms a smooth batter.
Drop ¼ cup of dough about 3 inches apart on a lined baking sheet.
Bake for 35-40 minutes, until puffed and golden.
Remove from the oven and gently pierce each cream puff with a knife. Return to the oven, with the door open for 20 minutes. After 20 minutes, remove from the oven and cool completely.
While they are cooling, prepare the whipped cream. Using a stand mixer whisk the heavy cream until soft peaks form. Add cocoa powder, powdered sugar and vanilla. Beat until stiff peaks.
Once the puffs are cool, using a serrated knife, cut the top ⅓ off each puff. Fill each puff with a scoop of whipped cream. Place the top back on.
Dust with powdered sugar or prepare the chocolate ganache.
Heat the heavy cream in a microwave safe bowl for 45 seconds. Add the chocolate chips and let it stand for 2 minutes. Stir until the chocolate chips are completely melted.
Spoon the ganache over the top of the prepared cream puffs. Let stand for about an hour for the chocolate to set up before serving.
