Preheat your oven to 350°F (175°C) and prepare a 9x13-inch baking pan by greasing it or lining it with parchment paper.

In a large mixing bowl, combine the yellow cake mix, melted unsalted butter, and large egg. Stir until it forms a thick batter, about 2-3 minutes.

Press 2/3 of the thick cake batter into the bottom of the prepared baking pan.

Sprinkle semi-sweet chocolate chips and toffee bits evenly over the pressed base layer. Optionally, add chopped pecans or walnuts on top.

In a separate bowl, whisk together the sweetened condensed milk, vanilla extract, and salt until smooth. Pour this layer over the toppings in the baking pan.

Crumble the remaining cake batter over the top of the condensed milk mixture.

Bake for 25 to 30 minutes until the edges turn golden-brown and the center is set. Keep an eye on it after 25 minutes.

Let the bars cool completely in the pan on a wire rack, about 1-2 hours. Once cool, slice into 15 squares.