Preheat oven to 350 degrees F.
Line a 8 x 8-in baking pan with parchment paper and grease with butter or nonstick spray.
In a large bowl, whisk melted butter and sugar. Whisk in eggs and yolk, one at a time, until smooth. Add vanilla. Gently mix in flour, cocoa powder, salt, and baking soda.
Pour batter into prepared baking pan and bake 30-35 minutes. Remove from oven and cool completely to room temperature.
Refrigerate 30 minutes, and then slice into 16 pieces.
To make the ghost topping, add 1 marshmallow to each brownie. Spoon 1-1 ½ tbsp melted white chocolate over marshmallow to coat. Allow chocolate to drip over the edges. Add 2 mini chocolate chips for eyes and let chocolate set completely.