Preheat your conventional oven to 350°F (175°C) and line a 9x9 inch baking pan with parchment paper, leaving an overhang on the sides for easy removal.
In a large bowl, combine the dry yellow cake mix, the melted butter, and the egg. Stir with a spatula until a thick, uniform dough forms.
Press approximately two-thirds of the dough firmly into the bottom of the prepared pan to create an even base layer.
Sprinkle the chocolate chips and the toffee bits evenly over the top of the pressed dough layer.
Drizzle the sweetened condensed milk slowly and evenly over the chocolate and toffee layer.
Take the remaining dough and crumble it into small, uneven pieces over the top of the condensed milk, creating a rustic, cobbler-like topping.
Bake for 25 to 30 minutes, or until the edges are golden brown and the center is set but still slightly soft to the touch.
Remove from the oven and allow the bars to cool completely in the pan on a wire rack before lifting them out using the parchment paper handles.
Cut into 16 equal squares using a sharp knife once the chocolate has fully set.