Preheat your oven to 350°F (175°C) and grease a 9×13-inch baking dish.
In a large bowl, combine the yellow cake mix, melted butter, and eggs. Stir until a thick dough forms.
Press two-thirds of the dough evenly into the bottom of the prepared baking dish.
Sprinkle the toffee bits and chocolate chips evenly over the dough.
Pour the sweetened condensed milk evenly over the top.
Crumble the remaining dough over the top of the toffee and chocolate layer.
Bake for 25–30 minutes, or until the top is golden brown and the bars are set.
Allow the bars to cool completely before cutting into squares and serving.