1. Preheat oven to 350°F and line a 9×9-inch pan with aluminum foil and spray with cooking spray; set aside.

2. To a large bowl, add the butter and beat with a handheld electric mixer on medium-high speed until smooth and creamy.

3. Add the cake mix and egg, and beat on low speed to combine. The batter will be very thick; keep beating until it becomes a tacky, thick dough.

4. Add between half and two-thirds of the dough to the prepared pan to form an even, smooth base layer. Use a spatula or your hands (sprayed with cooking spray) to press it down. Set the remaining dough aside.

5. Evenly sprinkle the white chocolate chips over the dough.

6. Evenly drizzle about 8 ounces of sweetened condensed milk over the top; just eyeball it.

7. Add the remaining dough in 1-inch pieces over the condensed milk, lightly pressing them down. It’s okay if you don’t have full coverage.

8. Bake for about 27 to 30 minutes, or until the edges are set and the center is mostly set. It’s okay if it’s a little jiggly; edges should be light golden brown.

9. Allow the pan to cool on a wire rack for at least 2 to 3 hours, preferably overnight, until the bars have firmed up enough to cut.