Preheat your oven to 350°F (175°C). This ensures that your bars bake evenly and achieve that lovely golden color.
Line a 9×9 inch pan with foil, leaving some overhang. This will make removing the bars much easier later. Spray the foil generously with cooking spray to prevent sticking.
In a bowl of an electric mixer fitted with a paddle attachment, combine the chocolate cake mix, egg, and unsalted butter. Mix on medium speed until the dough comes together and forms a cohesive ball. If you don't have a stand mixer, a hand mixer works just fine, though it may take a little longer.
Take about two-thirds of the dough and press it firmly into the bottom of the prepared pan, forming the crust. Make sure it's even so that your bars bake uniformly.
Evenly sprinkle the chocolate chips and Andes Peppermint Crunch Baking Chips over the crust you just made. This will create a delicious layer of chocolatey goodness.
Pour the sweetened condensed milk over the chips, covering them well. This will add that rich, gooey quality that makes these bars so amazing.
Now, drop pieces of the remaining dough over the top of the pan, using your hands to flatten them slightly. This doesn’t need to be perfect, but try to cover the surface as evenly as possible. Press gently to adhere.
Bake in the preheated oven for 23 to 25 minutes. You’ll know they’re ready when the edges turn golden brown and the top isn’t shiny anymore. Keep an eye on them to avoid overbaking.
Once baked, remove the pan from the oven and immediately sprinkle the coarsely crushed candy canes on top. They will melt slightly into the bars for an extra festive touch. You can omit this step if you'd prefer to keep them simple.
Allow the bars to cool completely in the pan. For a quicker cool-down, you can chill them in the refrigerator. Once cool, lift the bars out of the pan using the foil overhang and cut them into squares.
