01 -
Start by preheating your oven to 350°F. Line a 9×9-inch pan with aluminum foil (this will make cleanup so much easier!) and give it a good spray with cooking spray.
02 -
In a large bowl, beat the soft butter until it's smooth and creamy using a handheld electric mixer on medium-high speed.
03 -
Add your cake mix and egg to the butter, beating on low speed. Don't worry if the batter seems super thick - keep mixing until those pea-sized crumbles turn into a tacky, thick dough.
04 -
Press about two-thirds of your dough into the pan to create a smooth base layer. Sprinkle the white chocolate chips evenly over top, then drizzle with the sweetened condensed milk. Break up the remaining dough into 1-inch pieces and scatter them over the top, pressing down lightly.
05 -
Bake for 27-30 minutes until the edges are set and the center is mostly set (a little jiggle is fine!). You'll know they're done when the edges turn a light golden brown.
06 -
Let the bars cool on a wire rack for at least 2-3 hours, though overnight is even better. They need this time to firm up properly for cutting.