Instructions:
- Preheat oven to 350°F (175°C). Line a 9x13 inch pan with parchment paper, leaving an overhang on the long sides for easy lifting later.
- Make the Base Dough: In a large bowl, combine the cake mix, softened butter, and egg. Mix until a thick, crumbly dough forms.
- Press the Base: Press about two-thirds (2/3) of the dough mixture evenly into the bottom of the prepared pan to form a compact base layer.
- Create the Toffee Layer: Evenly sprinkle the butterscotch/toffee chips over the base layer. If using nuts, sprinkle those over the chips. Pour the sweetened condensed milk slowly and evenly over the top.
- Apply the Crumble Topping: Take the remaining one-third (1/3) of the dough mixture and lightly crumble it evenly over the toffee/milk layer. Do not press down hard.
- Bake for 30 to 35 minutes. The edges should be golden brown, and the center should look bubbly but set.
- Add Chocolate Topping: Immediately upon removing from the oven, sprinkle the semi-sweet chocolate chips evenly over the hot surface. Let it sit undisturbed for 5 minutes to melt the chocolate.
- Spread & Chill: Gently spread the melted chocolate into an even layer using a spatula. Sprinkle lightly with flaky sea salt. Allow the bars to cool completely on a wire rack, then chill for at least 2 hours before slicing for clean cuts.