Preheat oven to 350F. Line a 9x9” pan with foil and spray really well with cooking spray.
Place cake mix, egg, and butter in the bowl of an electric mixer fitted with a paddle attachment. (You can also use a hand mixer, it will just take longer.) Mix until the dough forms into a ball.
Place about 2/3 of the dough in the bottom of the pan and press to form the crust.
Sprinkle chocolate chips and Andes Baking Chips over the top. Pour sweetened condensed milk over the chips.
Drop pieces of the remaining dough evenly over the top of the pan. Press lightly to adhere.
Bake for 23-25 minutes until the edges get golden and the cake top is no longer shiny. Remove from oven and immediately sprinkle with crushed candy canes. Note: the candy canes will slightly melt into the top of the bars. Not all of them will stick. This is optional, and just to make them look pretty. You can omit the candy canes. (You can’t bake the candy canes on top or they will melt.)
Cool completely, chilling if needed to speed up the process, then cut into bars.