**Preheat the Oven:**
- Preheat your oven to 350°F (175°C). Grease a 9x9-inch baking dish lightly with butter or non-stick spray.
**Prepare the Cake Batter:**
- In a medium bowl, whisk together the flour, cocoa powder, sugar, baking powder, and salt.
- Add the milk, melted butter, and vanilla extract. Stir until well combined and smooth.
- Spread the batter evenly into the prepared baking dish.
**Make the Topping:**
- In a small bowl, mix the granulated sugar, brown sugar, and cocoa powder. Sprinkle this mixture evenly over the cake batter.
**Add the Hot Fudge Sauce:**
- Carefully pour the boiling water over the entire surface of the cake. Do not stir; the water will sink and form the fudgy layer during baking.
**Bake:**
- Bake for 35–40 minutes, or until the top is set and the cake begins to pull away from the edges of the dish. The bottom will remain gooey and fudgy.
**Serve:**
- Let the cake cool for 5–10 minutes. Serve warm with a scoop of ice cream or a dollop of whipped cream.
