Preheat oven to 350°F. Place butter in a 9x13 baking dish and put in preheated oven to melt. Remove and set aside.
In a bowl, mix together the sugar, cocoa powder, flour, milk and vanilla for the batter. Spoon batter into baking dish over melted butter. Do not stir.
In a separate bowl, mix together 2 cups sugar and 1/2 cup cocoa powder for topping. Sprinkle evenly over batter in pan.
Pour the boiling water carefully over the top of the batter and topping. Do not stir.
Bake for 35-40 minutes until top is set. Bottom will still be a little loose and saucy.
Allow to cool for 15-20 minutes before serving warm, topping with vanilla ice cream if desired.