Prepare 1 package white cake mix as directed on package. Pour into greased and floured 9 inch pan. Combine 1/2 cup brown sugar, 1/2 cup granulated sugar and 1/4 cup cocoa. Mix well. Spread evenly over cake batter. Pour 1 cup cold water over surface. Bake in 350 degree oven for 45 minutes. Serve warm.