Preheat the oven to 350°F (175°C) and lightly grease an 8-inch or 9-inch square baking dish.

In a medium mixing bowl, whisk together the all-purpose flour, granulated sugar, 2 tablespoons of cocoa powder, baking powder, and salt.

Stir in the whole milk, melted butter, and vanilla extract until the mixture forms a smooth batter.

Spread the batter evenly into the bottom of the prepared baking dish using a spatula.

In a small bowl, stir together the light brown sugar and the remaining 2 tablespoons of cocoa powder until well combined.

Sprinkle the sugar and cocoa mixture evenly over the top of the cake batter.

Slowly and carefully pour the boiling water over the entire surface of the mixture; do not stir or agitate the layers.

Bake for 35 to 40 minutes or until the cake portion on top is set and the sauce underneath is bubbling around the edges.

Remove from the oven and allow the dessert to stand for 15 minutes to allow the hot fudge sauce to thicken properly before serving.