Preheat oven to 350 degrees F and line a 9×13 inch baking dish with parchment paper.
Place cake mix, butter and egg into a medium mixing bowl. Stir until dough forms. Press into prepared baking dish. Drizzle with sweetened condensed milk, brown sugar, cinnamon, white chips and marshmallows./li>
Bake for 30-35 minutes, until baked through. Remove from oven and let cool completely before cutting into squares.