Preheat oven to 350°.
In a large bowl, whisk flour, sugar, 2 tablespoons cocoa, baking powder and salt.
In another bowl, whisk milk, oil and vanilla until blended.
Add to flour mixture; stir just until moistened.
Transfer to an ungreased 9-in.-square baking pan.
In a small bowl, mix brown sugar and remaining cocoa; sprinkle over batter.
Pour hot water over all; do not stir.
Bake 35-40 minutes.
Serve warm. Top with ice cream, if desired.