1. Preheat the oven to 350°F.
2. In a robot bowl (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 piping bag with a simple nozzle.
6. On a baking sheet covered with parchment paper, form small mounds 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 lemon cream, whisk the eggs with the sugar.
10. Add the lemon juice and cook over a bain-marie until a slightly thick cream is obtained. Incorporate the butter little by little.
11. Let cool, stirring from time to time to prevent a skin from forming on the surface of the cream.
12. Fill the whoopies with lemon cream and top with another one.