Heat oven to 425°F.
In a medium saucepan heat water, oil, sugar, and salt over medium heat until simmering. Add in flour and quickly stir with a wooden spoon until the mixture comes together and pulls away from the sides of the pan. Remove pan from heat and allow to cool down, about five minutes. Stir frequently (this helps to release the steam).
Add one egg and stir until completely incorporated. Add in remaining eggs, one at a time, until the mixture is smooth.
Add in cocoa and gently stir until smooth.
Using a pastry bag with a 2D tip, pipe out small clouds of the cocoa dough. You should have enough dough to pipe out 8 on one parchment-lined cookie sheet. (If you prefer you can use two soup spoons and drop 8 equally portioned mounds of the dough onto your prepared pan.)
Bake until completely puffed, 15 to 20 minutes. Reduce heat to 375 and continue to bake until shells are firm, about 15 more minutes.
Remove pan from oven and carefully make a small slit in the side of each puff. Allow puffs to dry out; let them sit out for about an hour. Allow puffs to cool completely before filling.