Prepare cake mix according to package instructions.
Spread half the cake batter in a greased 9x13 baking dish.
Sprinkle all the cinnamon crumble mixture over the top of the batter, then spread the remaining batter on top.
Bake according to the package instructions.
Allow the cake to cool completely, then whisk together the sweetened condensed milk, brown sugar, and cinnamon.
Poke holes all over the cooled cake and pour the condensed milk mixture over the top
Cover the cake pan and place in the refrigerator to chill for at least one hour.
When ready to serve, beat together the butter, cream cheese, powdered sugar, vanilla extract, and one tablespoon milk. Add more milk, if needed, until the desired consistency is reached.
Spread the icing over the top of the prepared cake before serving and garnish with more cinnamon, if desired.