Stir together flour, ¾ cup sugar, baking powder, and salt in an ungreased 9x13 casserole dish.
In bowl, add butter and ¼ cup of cocoa powder and stir together until smooth. Add to baking dish along with ½ cup milk and vanilla. Stir well with a fork.
In a small bowl, stir together remaining sugar and cocoa and sprinkle over the batter. Pour the 2 ½ cups hot milk over top. Do not stir.
Bake for 40 minutes or until sauce bubbles and cake begins to pull away from the side. Let cool. Serve warm.