Beat the butter with both sugars until creamy. Add the egg and mix until incorporated, then do the same with the milk.
Add the flour, cocoa powder, baking powder, baking soda, and salt all at once. Mix until just combined and transfer the batter to a piping bag
Line two baking sheets with parchment paper and pipe small mounds of batter. Mine were about 4-5 cm (1½-2 inches) in diameter. Leave enough space between them, as they will spread in the oven. You should end up with 40 cakes to make 20 whoopie pies.
Bake the cakes for 10-12 minutes at 180°C/350°F (conventional oven).
Raspberry filling + assembly
Place 150 grams powdered sugar, the butter, heavy cream, and the 3 raspberries in a bowl and mix until smooth and creamy. Add more powdered sugar if needed to make the filling firmer. Keep in mind that the filling will also firm up slightly as it rests.
Once the cakes are completely cooled, sort them by size. Transfer the filling to a piping bag and cut the extra raspberries in half.
Take one cake and pipe a ring of filling on top. Place half a raspberry in the center. Top with a second cake and your whoopie pie is ready. Repeat with the remaining cakes.
Tips
It works fastest if you place half of the cakes flat-side up. This way, you can pipe the filling onto all of them in one go and then assemble them.
Despite the heavy cream, the cakes can be stored outside the refrigerator, the sugar acts as a preservative.
How to store
Well wrapped, they keep for 4-5 days at room temperature, or up to 3 months in the freezer.