Step 1: Preheat
Preheat the oven to 350°F (175°C).
Step 2: Mix Dry Ingredients
In a bowl, mix flour, sugar, cocoa, baking powder, and salt. Add milk, melted butter, and vanilla. Stir in nuts if desired. Spread the batter evenly in an ungreased 9-inch square baking pan.
Step 3: Prepare Topping
In a separate bowl, combine brown sugar and cocoa. Sprinkle this mixture over the batter in the pan.
Step 4: Pour Water
Carefully pour hot water over the top