Preheat oven to 350°F.
Place the butter in a 9×13-inch baking dish and let it melt in the oven.
In a bowl, mix the flour, sugar, and cocoa powder. Stir in the milk and vanilla until smooth.
Spoon the batter over the melted butter. Do not stir.
Mix the topping sugar and cocoa powder together, then sprinkle evenly over the batter.
Slowly pour the boiling water over the top. Do not stir.
Bake for 30 minutes, or until the top is set.
Serve warm and enjoy!
