Melt butter: Place butter in a 9×13-inch baking dish and melt in the oven. Remove and set aside.
Make batter: In a bowl, combine the 1 1/2 cups sugar, 4 tablespoons cocoa powder, flour, milk, baking powder, salt, and vanilla extract. Spoon over melted butter in baking dish.
Make topping: In a separate bowl, mix together the 2 cups sugar and 1/2 cup cocoa powder. Sprinkle over batter in the pan. Pour boiling water over the topping.
Bake: Bake until the top has set, 35-40 minutes. The bottom will be loose and gooey.
Serve: Let cool 15 minutes before serving. Enjoy with vanilla ice cream.
