Preheat your oven to the temperature designated on the cake mix bow. Grease a 9-inch x 13-inch pan with cooking spray and set aside.
Prepare the cake batter and pour it into the prepared pan. Bake the cake according to package directions for a 9x13-inch pan.
Remove from the oven and poke 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 then pour all over the top of the cake. Allow the cake to cool and soak at room temperature for 2 hours.
Spread thawed whipped topping all over the top of the cake.
Combine the cinnamon and sugar in a small bowl and sprinkle over the top of the cake.
Notes
You can substitute a homemade white cake recipe if you prefer.
Store any leftover cake covered in the fridge for up to 2 days.