Preheat the oven to 350 degrees. Spray a 9 x 13 baking dish with cooking spray.
Prepare the cake according to the package directions and pour into the pan.
Bake for 25 minutes. Remove from oven and cool for 20 minutes.
Meanwhile, whisk together the sweetened condensed milk, butter, brown sugar, cinnamon, and salt. Mix well.
Poke holes using the bottom of a wooden spoon all over the cake. Pour a quarter of the milk and brown sugar mixture over the top and smooth it into the holes. Repeat until all the filling is gone.
To make the frosting beat the butter and cream cheese in a large bowl using a mixer. Add in the powdered sugar and vanilla and beat until smooth.
Spread over top of the cake. Sprinkle with cinnamon.
Refrigerate for at least 2 hours or up to 3 days.