Preheat the oven to 350°F (175°C) and grease a 9-inch square baking pan.
In a medium mixing bowl, whisk together the flour, granulated sugar, cocoa powder, baking powder, and salt until evenly combined.
Add the milk, vegetable oil (or melted butter), and vanilla extract to the dry ingredients. Stir with a spatula until the batter is smooth and thick.
Transfer the batter to the prepared baking pan and spread evenly.
In a separate bowl, mix the granulated sugar, brown sugar, and cocoa powder for the hot fudge sauce.
Evenly sprinkle the dry sugar mixture over the cake batter in the pan. Do not stir.
Carefully pour the boiling water over the entire surface of the batter. Do not stir.
Place the pan in the preheated oven and bake for 35-40 minutes, or until the cake is set on top but still slightly soft in the center.
To check for doneness, insert a toothpick into the cake layer. It should come out clean while the bottom remains gooey.
Once baked, remove the cake from the oven and let it sit for 10-15 minutes.
Scoop portions of the cake, ensuring to get plenty of the hot fudge sauce from the bottom.
Serve warm, drizzling extra sauce over the top.