Preheat your oven to 350 degrees Fahrenheit
Whisk together the baking powder, flour, sugar, salt, and two tablespoons of cocoa in a bowl.
Combine milk, vanilla, and oil in another bowl until they blend together.
Combine the wet and dry ingredients and mix together.
Pour mixture into an ungreased baking pan (about 9 inches will do).
Mix together the rest of the cocoa and your brown sugar. Sprinkle that over the contents of the baking pan.
Pour hot water over the entire pan but do not mix it in.
Bake the cake batter for 35-40 minutes and serve it hot.