Directions for the cake: Mix together the sugar, flour, cocoa, baking powder and salt in a large bowl. Stir in milk, melted butter and vanilla until combined. Spread the batter over the bottom of a large crockpot.
For the fudge topping: Mix together the brown sugar and cocoa. Sprinkle mixture over the cake batter.
Pour the boiling water over the top. DO NOT STIR! Cover and cook on High for 2 - 2 1/2 hours.
Cake will be very hot! Turn off heat, and remove lid. Let sit 20-30 minutes before serving, so its still warm but wont burn your mouth : ) ENJOY!!