Preheat oven to 350°F (175°C). Lightly grease a 9-inch square baking pan.

Combine 1/2 cup granulated sugar with flour, cocoa powder, baking powder, and salt. Add milk, butter, and vanilla; whisk until smooth and fairly lump-free.

Pour the batter into the prepared baking pan and spread evenly.

Combine remaining 1/2 cup white sugar, brown sugar, and remaining 3 tablespoons cocoa powder. Evenly sprinkle mixture over the batter.

Slowly pour boiling water over the top; do not stir or mix into the batter; allow the water to sit on top.

Bake for 35 minutes or until the center is almost set. Remove from oven and allow to cool for 15 minutes.

Serve in dessert dishes with ice cream or whipped cream, spooning fudge sauce from the bottom of the pan over each serving.