01 -
Set your oven to 350°F before you do anything else. To make cleanup simple, cover a 9x9-inch pan with foil, then give it a good spray with cooking spray so nothing sticks.
02 -
In a big bowl, grab your electric hand mixer and beat the butter until it's smooth like frosting. Medium-high speed works great.
03 -
Throw in the cake mix and the egg with the butter, and switch your mixer to low speed. Don't be alarmed by the super thick dough—it’ll eventually come together and feel sticky and a bit lumpy with pea-sized bits.
04 -
Push about two-thirds of your dough into the bottom of the pan to form the base layer. Scatter the white chocolate chips all over evenly, then drizzle the sweetened condensed milk across the top. Lastly, tear the leftover dough into small pieces (about 1-inch chunks), placing them over the top, and press everything down lightly.
05 -
Bake in the oven for around 27 to 30 minutes. The edges should be set while the center can stay just a tad wobbly. Look for a nice, light golden brown along the edges to know it’s ready.
06 -
Let the bars rest on a cooling rack for 2 to 3 hours. If you can, give it a full overnight rest—it’ll make slicing them so much easier once everything’s firmed up.