Spray a 4-6 quart slow cooker with cooking spray.
In a bowl mix together the flour, sugar, 4 tablespoons of cocoa powder, baking powder and salt. Then stir in the milk, vegetable oil and vanilla until combined. Spread batter evenly into the bottom of the greased slow cooker.
Combine the brown sugar, hot water, and the remaining 1/2 cup cocoa powder and whisk together until smooth. Pour over the batter in the slow cooker. DO NOT MIX!
Cover and cook on high for 2 hours.
Dish chocolate cake onto dessert dishes and then drizzle the hot fudge sauce from the bottom of the slow cooker over the top. Serve with your favorite sundae toppings such as: caramel, sprinkles, nuts, whipped cream, or cherries or even a scoop of ice cream.