1. Preheat the oven to 350F.
2. In the bowl of a robot (or with an electric whisk), whisk the butter and sugar for 4 minutes. Add the whole egg and vanilla; mix.
3. Sift the flour, baking powder, cocoa powder, baking soda, and salt. Add to the preparation.
4. Add the milk, whisking slowly.
5. Transfer the preparation to a pastry bag with a simple nozzle.
6. On a baking sheet covered with parchment paper, dress small piles of dough spaced apart.
7. Bake for 10 minutes.
8. Remove from the oven and let the whoopies cool on a rack.
9. For the cream: mix the butter with the powdered sugar and water. Add the vanilla extract.
Assemble the whoopies with cream and top with another whoopie to close. Serve.