01 - Preheat oven to 350°F. Line an 8x8-inch baking pan with parchment paper and lightly grease.
02 - In a mixing bowl, whisk together melted butter and sugar until combined. Add eggs and vanilla extract; mix until smooth.
03 - Sift in cocoa powder, flour, salt, and baking powder. Stir just until incorporated, avoiding overmixing.
04 - Spread brownie batter evenly in the prepared pan, smoothing the surface.
05 - Bake for 25 to 30 minutes or until a toothpick inserted into the center yields a few moist crumbs. Allow brownies to cool completely in the pan.
06 - Slice brownies into 16 squares once cooled.
07 - Place one marshmallow in the center of each brownie square.
08 - Place white chocolate chips in a microwave-safe bowl. Heat in 20-second intervals, stirring after each, until fully melted and smooth.
09 - Spoon or pipe melted white chocolate over each marshmallow, allowing it to drip for a ghostly effect.
10 - While chocolate is soft, use black decorating gel or melted dark chocolate to dot eyes and a mouth for each marshmallow ghost.
11 - Allow chocolate to set before serving.