Preheat the oven to 350?F.
In a medium bowl, stir together the flour, sugar, cocoa, baking powder and salt. In a small bowl or measuring cup, stir together the milk, oil and vanilla; add to the dry ingredients and stir just until combined. Spread into a greased or parchment-lined 9-inch square pan.
In the same bowl, stir together the sugars and cocoa, then stir in the hot water or coffee; pour over the batter but don’t stir it. Bake for 20 minutes, or until the cake is springy to the touch and there is a thick chocolate sauce underneath. Serve warm, with vanilla ice cream. Serves 6.