Meanwhile, heat 1/4 cup butter, the milk and 3 tablespoons baking cocoa in 2-quart saucepan over medium heat until butter is melted and mixture is smooth. Stir in 1/2 teaspoon vanilla and the powdered sugar until thick yet pourable. Pour over marshmallow layer, spreading with spatula to cover marshmallows. Cool completely on cooling rack, about 2 hours. Cut into 4 rows by 4 rows.