1. Preheat the oven to 350F.
2. In a mixer, beat the butter and sugar together for 4 minutes.
3. Add the whole egg and vanilla, mix and add the milk.
4. Add the flour previously mixed with the baking powder, bicarbonate, and salt.
5. Transfer the mixture to a pastry bag with a simple nozzle.
6. On a plate with parchment paper, form small heaps of dough spaced apart.
7. Bake for 10 minutes. Let cool on a rack.
8. Fill half of the biscuits with a marshmallow and top with another biscuit.
9. Pass through a hot oven for 3 minutes to soften the marshmallow.
10. For the glaze: mix the powdered sugar and water.
11. Brush the top of the whoopies and let rest for 5 minutes before serving.