In a bowl combine the flour, granulated sugar, 2 Tbsp cocoa powder, baking powder and salt; stir until evenly combined. Add in the vegetable oil, vanilla, and milk, whisk to combine. Pour into an UNGREASED 9x13 inch pan and spread evenly.
In another bowl combine the brown sugar and the ¼ cup cocoa powder, stir until evenly combined. Sprinkle over the batter in the pan.
Pour the hot water over the top in the pan and place in a preheated 350°F oven. Bake for 40 minutes. The water will have combined with the brown sugar/cocoa powder mixture and seaped to the bottom to create hot fudge and forcing the batter to move to the top which when baked with create the cake. Serve immediately with your favorite ice cream.