Preheat oven to 350°.
For the batter: In a large bowl, whisk together the sugar, flour, cocoa, baking powder, and salt.
Stir in the milk, melted butter and vanilla, mixing until smooth.
Pour into an ungreased 8-inch square glass baking pan, (or something comparable… I have a 7×11 dish that is perfect for this!)
For the topping: In a small bowl, whisk together the sugar, brown sugar, and cocoa. Sprinkle evenly all over the top of the batter.
Gently pour the hot water over the top of the topping. Do not stir! (Yes, it just looks like a big puddle of chocolate water on top.)
Bake for 35-40 minutes, or until center is almost set.
Let cool for 10-15 minutes, then serve. Spoon the hot fudge sauce in the bottom of the pan over the cake and serve with ice cream, whipped cream or heavy cream.