Preheat oven to 350 degrees and line a 9×13 inch baking pan with parchment paper.
Place melted butter, cake mix, Biscoff and egg into a large mixing bowl. Mix until dough forms and stir in chocolate chips. Press dough into prepared baking pan. Bake for 25-28 minutes or until baked through. Let cool for 20 minutes.
To prepare icing, heat sweetened condensed milk and Biscoff spread into a small saucepan over medium low heat until creamy and thick. Remove from heat and let sit for 5 minutes. Pour over bars and cut into squares. Serve gooey or let icing set up.