Preheat the oven to 350°F (175°C). Grease a 9x13-inch baking dish or line it with parchment paper.
Prepare the crust: In a large bowl, mix the cake mix, melted butter, and egg until combined. Press the mixture evenly into the bottom of the prepared baking dish.
Layer the toppings: Sprinkle the toffee bits and chocolate chips evenly over the crust.
Add the sweetened condensed milk: Drizzle the condensed milk evenly over the entire surface.
Bake: Place in the oven and bake for 25 minutes, or until the edges are golden brown and the center is set.
Cool & slice: Let the bars cool completely before slicing into squares. Serve and enjoy!