Preheat oven to 350 degrees F. Line a 13x9" pan with foil, extending sides of the foil over the edge of the pan. Spray the foil lightly with cooking spray.
In a large bowl, combine the cake mix, egg and melted butter with a spoon. Press the mixture evenly into the prepared pan and bake for approx. 15-18 minutes or until a toothpick inserted in the center comes out clean. Cool completely.
In the bowl of a stand mixer, beat together the softened butter, peppermint stick flavor packet, milk + powdered sugar until combined. Spread the filling mixture evenly over the cooled bars. Allow the filling to set, about 30 minutes-1 hour in the fridge.
Melt the chocolate chips and Tablespoon of oil in the microwave until smooth. Spread the chocolate mixture evenly over the filling; sprinkle immediately with crushed candy canes. Allow the bars to set in the fridge for another hour or so before cutting into squares. These bars can be served up to 2 days later when stored in the fridge.
Recipe by The Domestic Rebel at https://thedomesticrebel.com/?p=3879