Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking pan or line it with parchment paper.

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

Set aside about one-third of the cake mixture for the topping and press the remaining two-thirds into the base of the prepared baking pan.

Pour the sweetened condensed milk over the pressed crust and spread it evenly.

Sprinkle the semi-sweet chocolate chips and toffee bits over the condensed milk layer evenly.

Crumble the reserved cake mixture over the chocolate and toffee layer for topping.

Bake for 25-30 minutes until the edges are golden brown and the center is soft.

Let the bars cool completely in the pan on a wire rack, then cut into squares or rectangles.