Preheat your oven to 350°F (175°C) and grease a 9×13-inch baking pan.

In a large bowl, combine the cake mix, melted butter, and egg until a soft dough forms.

Press two-thirds of the dough evenly into the bottom of the prepared baking pan.

Sprinkle the toffee bits evenly over the dough layer.

In a small saucepan over low heat, combine the sweetened condensed milk, chocolate chips, and vanilla extract. Stir until melted and smooth.

Pour this mixture evenly over the toffee bits.

Crumble the remaining dough over the chocolate layer.

Bake for 25-30 minutes, or until the top is golden brown.

Allow the bars to cool completely before slicing into squares.