Spray the slow cooker with baking spray. Mix brown sugar, flour, cocoa, baking powder, and salt in a mixing bowl. Stir in milk, butter, and vanilla. Spoon batter into bottom of slow cooker; dough will be very thick.
In another bowl, mix together cocoa and brown sugar. Sprinkle evenly over batter. Do not stir. Pour boiling water over the top. Do not stir. Cover and cook on HIGH for 2-1/2 to 3 hours or until the cake pulls away from the sides of the slow cooker (pudding will be bubbling through). Serve immediately with ice cream or whipped topping, if desired. Or let set for 10 minutes and then invert onto a plate. Spoon pudding left in the pot over the cake.