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.