Preheat the Oven: Preheat your oven to 350°F (175°C).
Mix Dry Ingredients: In a large mixing bowl, combine flour, granulated sugar, 2 tablespoons cocoa powder, baking powder, and salt.
Add Wet Ingredients: Add the milk, vegetable oil, and vanilla extract to the dry ingredients. Stir until just combined—the batter will be thick.
Prepare the Topping: In a separate bowl, mix together the brown sugar and ½ cup cocoa powder.
Assemble the Dish: Pour the thick batter into a greased 9x9-inch baking dish. Sprinkle the brown sugar and cocoa mixture evenly on top of the batter.
Pour Hot Water: Carefully pour the hot water over the entire dish; do not stir. This will create the fudge layer at the bottom.
Bake: Bake in the preheated oven for 30-35 minutes or until the top is set and the center is still slightly wobbly.
Serve: Let it cool for a few minutes before serving with whipped cream or ice cream if desired.