Preheat the oven to 350°F and line a 9×9-inch baking pan with aluminum foil for easy cleanup, then lightly coat with cooking spray; set aside.

In a large mixing bowl, beat the softened butter with a handheld electric mixer on medium-high speed until smooth and creamy.

Add the cake mix and egg, then mix on low speed until fully combined. The batter will be very thick, starting as pea-sized crumbles before forming a tacky, dense dough.

Press about half to two-thirds of the dough evenly into the prepared pan to create a smooth base layer. A spatula works well, but you can also use your hands coated with cooking spray if easier; set aside the remaining dough.

Sprinkle the white chocolate chips evenly over the dough layer.

Drizzle approximately 8 ounces (about 60% of the can) of sweetened condensed milk evenly over the white chocolate chips.

Tear the remaining dough into roughly 1-inch pieces and scatter them over the condensed milk layer, gently pressing down. Some gaps are fine.

Bake for 27 to 30 minutes, or until the edges are set and lightly golden brown while the center remains slightly jiggly.

Remove from the oven and allow the pan to cool on a wire rack for at least 2 to 3 hours, or preferably overnight, until the bars have fully firmed up for easy slicing.