To make the cake: In a medium bowl, sift together flour, cocoa, baking powder, and salt. Add sugar and mix.
To make the pudding: In a small bowl, toss together cocoa, granulated sugar, and brown sugar until combined. Sprinkle mixture on top of batter, making sure it coats the entire surface evenly. Pour the hot water evenly on top. Leave as-is and do not stir.