Prepare brownies in a 9x9 pan according to the box directions. Let them cool completely.
Line the pan with parchment paper to lift brownies out easily.
Once cool, cut brownies into 16 squares and place them on a parchment-lined baking sheet, spaced about an inch apart.
Melt the white chocolate wafers following package instructions.
Dip the bottom of a large marshmallow into melted white chocolate and press onto the center of each brownie.
Spoon melted white chocolate over each marshmallow, letting it drip down the sides.
Let the brownies sit until the chocolate sets and hardens.
Melt semi-sweet chocolate chips, then use a small Ziploc bag to pipe eyes and mouths on each ghost. Let chocolate set before serving.