Preheat your oven to 350°F (175°C). Grease a 9×13-inch baking pan or line it with parchment paper for easy slicing and removal later.

In a large mixing bowl, combine the yellow cake mix, melted butter, and egg. Stir until the mixture is crumbly but holds together when pressed.

Take about two-thirds of the mixture and press it evenly into the bottom of the prepared pan to form a crust.

Pour the sweetened condensed milk evenly over the crust. Use a spatula to spread it out gently, ensuring the entire surface is covered.

Sprinkle the semi-sweet chocolate chips and toffee bits evenly over the condensed milk layer. This step is what gives these bars their signature gooey-crunchy contrast.

Take the remaining cake mix mixture and crumble it over the top. Don’t worry about covering it completely—leaving some spots exposed allows the chocolate and toffee to peek through.

Bake for 25–30 minutes, or until golden brown around the edges and just set in the center. The middle may still look slightly gooey—that’s exactly what you want. Remove from the oven and let cool completely in the pan before slicing into bars.