Make Cookies: Whip butter and sugar until creamy. Add vanilla and milk. Sift in flour, cocoa, baking powder, and soda. Fold in white chocolate chips. Roll into 6 balls and flatten slightly.
Bake and Shape: Bake at 180°C for 12-15 minutes. Use a round cutter to scoot into a circle while warm. Cool completely.
Make Frosting: Whip butter until smooth. Add icing sugar and cocoa powder. Pour in aquafaba and whip on high speed until light and airy.
Make Marshmallow Ghosts: Melt white chocolate in a bain-marie. Drizzle over marshmallows on toothpicks. Freeze for 2-4 minutes to set.
Assemble: Pipe frosting on each cookie. Drizzle with white chocolate and place a marshmallow ghost in the center. Melt dark chocolate and pipe 3 dots for the ghost's eyes and mouth.