Preheat your oven to 350°F (175°C). Line a 9x13-inch baking dish with parchment paper or give it a light spray of nonstick cooking spray.
In a medium bowl, mix together the cake mix, melted butter, and egg until it turns into a soft dough. No need to overmix.
Press about two-thirds of the dough into the bottom of your prepared pan. Save the rest for topping.
Sprinkle the toffee bits and chocolate chips evenly across the base layer.
Drizzle the sweetened condensed milk all over the top. Don’t skimp—it’s the gooey glue that holds the magic together.
Drop spoonfuls of the remaining dough over the top. It won’t cover everything, and that’s totally okay.
Bake for 25 to 30 minutes, until the edges are golden and the center looks set. Cool completely before slicing. Patience, my friend.