Make the cake batter

In a large mixing bowl, combine ¾ cup of the granulated sugar, the all purpose flour, ¼ cup of the cocoa powder, the baking powder, and salt. Stir until evenly blended. 

Add the milk, melted butter, and vanilla extract to the dry ingredients. Mix well — use a hand mixer or whisk — until the batter is smooth and lump-free. 

Pour the batter into the prepared baking dish and spread it evenly. 

Preheat and prep pan

Preheat oven to 350°F (177°C). 

Spray a 9-inch square baking dish with cooking spray (or butter and flour it). Set aside. 

Prepare the topping that becomes the fudge sauce

In a small bowl, combine the remaining ½ cup granulated sugar, the remaining ⅓ cup cocoa powder, and the packed brown sugar. Stir to mix thoroughly. 

Sprinkle this sugar-cocoa mixture evenly over the cake batter in the pan. Do not stir — just let it sit on top of the batter. 

Add the hot water (the “magic” step)

Carefully and slowly pour 1 1/4 cups HOT water over the top of the sugar-cocoa layer. Pour gently and evenly so you don’t disturb the batter underneath. Do not stir. 

Bake

Place the pan in the preheated oven and bake for 32–35 minutes, or until the top cake layer looks mostly set but still slightly soft in the center. The sauce beneath will still be liquid — that’s how it should be. 

Once baking time is up, remove from oven and let stand for 10 minutes before serving. This resting time helps the sauce settle and thicken a little. 

Serve

Scoop into individual bowls so each serving has both cake and the gooey fudge sauce underneath.

For an indulgent finish, top with vanilla ice cream, whipped cream, or even a drizzle of extra hot fudge or caramel.