1. Preheat the oven to 350F.
2. In a mixing bowl (or 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, bicarbonate, and salt. Add to the mixture.
4. Add the milk, whisking slowly.
5. Transfer the mixture to a piping bag with a simple nozzle.
6. On a baking sheet covered with parchment paper, pipe small, spaced-out dollops of dough.
7. Bake for 10 minutes.
8. Remove from the oven and let the whoopies cool on a wire rack.
9. For the cream: mix the butter with the icing sugar and water. Add the vanilla essence.
10. Assemble the whoopies with cream and stack them to form small towers of 5 whoopies, placing them on top of each other with cream in between each one.
11. Decorate with chocolate chips and serve.