Preheat the oven to 350°.
Place the butter in a 9x13-inch baking dish and set it in the oven to melt.
In a mixing bowl, combine the sugar, self-rising flour, milk, vanilla, and cocoa, stirring until smooth.
Once the butter has melted, carefully spoon the batter evenly over the butter-do not stir.
In a separate bowl, mix together the sugar and cocoa for the topping.
Sprinkle this mixture evenly over the batter, again without stirring.
Slowly pour the boiling water over the top, making sure not to mix.
Return the dish to the oven and bake for 35-40 minutes.
Remove from the oven and let it cool for 15-20 minutes before serving.