Preheat oven to 350. In an ungreases 9x13 pan; stir in flour, sugar, 4 TBSP cocoa, baking powder, and salt. Combine with fork. Add milk, oil, and vanilla till smoothed out across bottom of pan to completely cover bottom.
From this point on...DO NOT STIR! Sprinkle top with the brown sugar and 1/2 cup cocoa. then slowly pour on top the hot tap water. Bake for 40 minutes. Let stand for 15. Serve warm with Ice cream!