Spray the inside of a 4-5 quart casserole dish with cooking spray.
In a large bowl, stir together flour, baking powder, salt, 6 Tbs. cocoa and 1 1/2 cup white sugar.
Stir milk, melted butter and vanilla into the flour mixture until smooth. Pour into the greased casserole.
In a small bowl, combine 1 cup white sugar, brown sugar and cocoa. Sprinkle over the top of the batter.
Pour hot water over the top of batter. Do not stir!
Bake at 350 degrees for 45 minutes.
Sprinkle with salt and let stand several minutes to allow the cobbler to thicken.