Pour the melted butter into a 9-by-13-inch baking dish.
In a mixing bowl, combine the self-rising flour, milk, vanilla and 1 1/2 cups of the sugar. Mix well and pour over the melted butter.
In another bowl, mix together the cocoa powder and remaining 1 cup sugar. Sift over the baking dish with the melted butter and flour mixture. Pour 3/4 to 1 cup of hot, hot water over top. Bake until firm on top but still gooey and chocolatey on the inside, 30 to 35 minutes.