Preheat your oven to 350°F (175°C). This ensures that your bars will bake evenly.
Grease a 9x13-inch baking dish with butter or line it with parchment paper for easy removal later.
In a large mixing bowl, cream together the softened unsalted butter and packed brown sugar until light and fluffy. Gradually add in the all-purpose flour and salt, mixing until the mixture resembles coarse crumbs.
Take about two-thirds of the crumb mixture and press it evenly into the bottom of the prepared baking dish to form the crust. Reserve the remaining crumb mixture for topping.
In a microwave-safe bowl, combine the bittersweet chocolate chips and sweetened condensed milk. Microwave in 30-second intervals, stirring in between, until the chocolate melts and the mixture is smooth.
Stir in the Andes Creme de Menthe Chocolate Mint Baking Chips into the melted chocolate mixture until well combined.
Pour the fudge filling over the crust in the baking dish, spreading it evenly. Sprinkle the reserved crumb mixture on top, followed by the semi-sweet chocolate chips and mint M&Ms.
Bake in the preheated oven for 25-30 minutes, or until the top is golden and the edges are set. Allow to cool completely in the pan on a wire rack.
Once cooled, cut into squares and serve. Enjoy the delightful combination of chocolate and mint in every bite!