Preheat oven to 350°.
Spray an 8" or 9" glass pan or casserole dish with cooking spray.
In a medium bowl, stir together 3/4 cup granulated sugar, flour, 3 tablespoons cocoa, baking powder and salt. Blend in milk, butter and vanilla; mix until smooth. Pour batter into prepared pan.
In a separate bowl, stir together remaining 1/2 cup granulated sugar, brown sugar and remaining 4 tablespoons cocoa; sprinkle mixture evenly over batter (it looks like a lot but that's the key).
Gently pour boiling water over top; do not stir.
Bake 35-40 minutes. The top of the cake should be mostly set when the pan is moved.
Remove from oven. Allow to cool at least 15 minutes.
Serve while warm with ice cream or whipped topping.