Heat oven to 350°F. Line a 9x13-inch pan with parchment or foil and lightly coat with cooking spray.
In a bowl, mix the cake mix, melted butter, and egg until a soft, thick dough forms.
Press about two-thirds of the dough evenly into the prepared pan to form the base.
Scatter the chocolate chips and toffee bits evenly over the crust.
Pour the sweetened condensed milk over the top, spreading gently to cover while avoiding the very edges.
Crumble the remaining dough over the filling in small pieces, leaving a few gaps.
Bake 25 to 30 minutes, until the top is golden and the center looks set.
Cool completely in the pan on a rack. Lift out using the liner and cut into bars.
Notes
Line the pan to make removing and slicing easier. Use any chocolate you like—milk, semisweet, or dark. Store the cooled bars airtight at room temperature for up to 3 days, or freeze up to 2 months.