1. Preheat the oven to 350F.
2. In a mixing bowl, beat 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 mixture.
4. Add the milk, mixing slowly.
5. Transfer the mixture to a pastry bag with a simple tip.
6. On a baking sheet lined with parchment paper, pipe small balls of dough, spaced apart.
7. Bake for 10 minutes.
8. Remove from the oven and let the whoopie pies cool on a rack.
9. For the cream: mix the butter with the powdered sugar and water. Add the food coloring and strawberry extract. Fill half of the whoopie pies with cream and top with another. Serve.