Preheat oven to 350F and line a 9×9-inch pan with aluminum foil (strongly recommended for easier cleanup) and spray with cooking spray; set aside.
To a large bowl, add the butter and beat with a handheld electric mixer on medium-high speed until smooth and creamy.
Add the cake mix, egg, and beat on low speed to combine. The batter will be very thick; keep beating until it goes from pea-sized crumbles to a tacky, thick dough.
Add between half and two-thirds of the dough to the prepared pan to form an even, smooth base layer. I use a spatula but use your hands (sprayed with cooking spray) if that’s easier; set remaining dough aside.
Evenly sprinkle the white chocolate chips over the dough.
Evenly drizzle with about 60% of the can (about 8 ounces) of sweetened condensed milk; just eyeball it.
Add the rest of the dough in approximately 1-inch pieces over the sweetened condensed milk, lightly pressing it down. You won’t have complete coverage; this is okay.
Bake for about 27 to 30 minutes or until edges are set and the center is mostly set; it’s okay if it’s a little jiggly. I judged the doneness by looking at the edges and as they turned light golden brown, I removed the pan from the oven. Allow pan to cool on a wire rack for at least 2 to 3 hours (overnight is better) or until bars have firmed up enough to cut. Bars are best fresh but will keep airtight at room temp for up to 5 days or in the freezer for up to 3 months.