Preheat oven to 350°F (175°C) and grease a 9×13-inch baking pan.
In a large bowl, mix the cake mix, melted butter, and egg until a 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, stir together the sweetened condensed milk, chocolate chips,
and vanilla extract until melted and smooth. Pour this mixture 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 and serving.
