For the cookies:

1. Preheat your oven to 350°F (180°C). Line two baking trays with parchment paper.
2. In a food processor, combine flour, cocoa powder, baking powder, and sugar. Add cold butter and process until the mixture resembles fine crumbs.
3. Add the egg, vanilla extract, and half of the milk. Process until the dough forms large clumps. Add the remaining milk if needed.
4. Lightly knead the dough on a clean surface until it holds together. Roll it out between two sheets of baking paper to about 4-5 mm thick and chill in the fridge for 30 minutes.
5. Cut out cookies using a 2-inch circle cutter. Place them on the baking trays about 1 inch apart. Re-roll and cut any remaining dough.
6. Bake for 6 minutes, then rotate the trays and bake for another 6 minutes. Cool on a wire rack.

For the buttercream filling:

1. In a cleaned food processor, blend powdered sugar, softened butter, heavy cream, vanilla extract, and salt until smooth and pale.
2. Divide the buttercream into two bowls. Color one batch with purple food coloring and the other with orange.
3. Place a teaspoon of buttercream on the center of half the cookies and sandwich with another cookie. Decorate with candy monster eyes and sprinkles.
4. Refrigerate if needed to firm up, or serve immediately.