Preheat the oven to 350°F. Spray a 6½-inch cast iron skillet with canola oil spray. Line a quarter-sheet baking pan with foil to catch any drips.
For the base: in a small bowl, sift the flour and cocoa powder together to remove any lumps. Add the baking powder, espresso powder (if using), and salt; whisk well.
Add the milk, sugar, butter, and vanilla extract and mix well.
Spread the batter into the prepared skillet, smoothing with a small offset spatula as you go.
For the top: in a small bowl, sift the cocoa powder to remove lumps. Add in the sugar and brown sugar; whisk well. Sprinkle the mixture over the batter in the pan.
Pour the hot water over everything, but don't mix it in...the water will create the hot fudge sauce without help from us. The water will come almost to the top of the skillet
Place the skillet on the foil-lined baking pan, and put in the oven. Bake for 25 to 30 minutes, or until the top brownie appears firm. Keep an eye on it towards the end of the baking time so it doesn't get too dark.
Remove the pan to a cooling rack and allow to cool for 15 minutes. Serve with a scoop of vanilla ice cream, grab a spoon with a friend, and enjoy!