1. Preheat the oven to 350F.
2. In the bowl of a stand mixer (or using an electric whisk), beat the butter and sugar for 4 minutes. Add the whole egg and vanilla extract; mix.
3. Sift the flour, baking powder, cocoa powder, baking soda, and salt. Add to the mixture.
4. Add the milk, whisking slowly.
5. Transfer the mixture to a piping bag fitted with a plain nozzle.
6. On a baking sheet lined with parchment paper, pipe small mounds of dough, spaced apart.
7. Bake for 10 minutes.
8. Remove from the oven and let the whoopies cool on a wire rack.
9. Fill half of the whoopies with raspberry jam and top with another whoopie. Serve immediately.