To a medium bowl, add all of the batter ingredients (flour, granulated sugar, cocoa powder, baking powder, salt, milk, butter, and vanilla extract). Whisk together until blended and smooth. Using a spatula, spread batter in the bottom of a Crock Pot.
In a medium bowl, whisk together the dry layer (granulated sugar, brown sugar, and cocoa powder). Sprinkle evenly over the top of the batter.
Slowly pour the hot water over the top.
Cook on high until cake has puffed up and it springs back when you poke it with your finger, 3 to 4 hours.
Use a spoon to scoop cake and sauce onto dessert plates along with a scoop of vanilla ice cream. Serve.
