Grease the inside of a 9-inch x 13-inch pan with cooking spray and set aside.
Prepare the cake batter and pour it into the prepared pan. Then bake the cake according to package directions for a 9x13-inch pan.
Remove the cake from the oven and poke the top all over with a fork, skewer, or back of a wooden spoon.
Whisk together the sweetened condensed milk and RumChata in a large measuring cup and pour all over the top of the cake. Allow the cake to cool and soak at room temperature for 2 hours.
Spread thawed Cool Whip whipped topping all over the top of the cake.
Combine the cinnamon and sugar in a small bowl and then sprinkle evenly over the top of the cake. Chill in the fridge until ready to serve.
Notes
Store leftover cake covered in the refrigerator for up to 2 days.Nutrition will vary based on the number of slices you cut into the cake.