Melt the butter in a small saucepan over low heat and let it cool to room temperature.

In a large bowl, whisk together the cooled melted butter, granulated sugar, and dark brown sugar until well blended. Add the eggs and vanilla extract, mixing until smooth.

In a separate bowl, whisk together the flour, cocoa powder, baking powder, baking soda, and salt. Gradually add this dry mixture to the wet ingredients, mixing gently until just combined.

Cover the bowl with plastic wrap and refrigerate the dough for 30 minutes. Preheat your oven to 350°F (175°C).

Scoop heaping tablespoons of dough and roll into balls. Place on parchment-lined trays, flatten slightly, and bake for 9-10 minutes.

Let the cookies cool on the tray for a few minutes, then transfer to a wire rack. Prepare the vanilla buttercream by mixing sifted powdered sugar with milk until smooth and creamy. Once cooled, pipe the buttercream on each cookie and add candy eyes.