Preheat oven to 350 degrees F. Line a 9x9 baking pan with parchment paper that has a slight overhang on 2 sides, then lightly spray with nonstick spray.
With an electric mixer, cream the butter until smooth and fluffy. Beat in the cake mix, egg, and lemon zest until combined and a thick dough forms.
Press 2/3 of the cake mixture in the bottom of the prepared pan. Spread the lemon curd evenly on top of the dough, but not completely to the edges.
Sprinkle the white chocolate chips over the lemon curd, then drizzle the sweetened condensed milk over the top of the chocolate (leave a little border bare, without sweetened condensed milk.)
Break up the remaining dough into 1/2 to 1 inches pieces and sprinkle it over the top of the bars (leave some of the sweetened condensed milk exposed so you have a cobbled look once baked.) Press down on the dough pieces lightly to seal.
Bake for about 29 to 32 minutes, until slightly browned. Cool completely, then make sure to chill for a few hours before cutting into bars.
Use parchment paper. I highly recommend lining your baking dish with parchment paper because the sweetened condensed milk is sticky. It makes removal so much easier.Leave a border of the bars without sweetened condensed milk. Be sure to leave a border around the edge of the uncut bars in the pan without and sweetened condensed milk so that it doesn’t bubble up and burn during baking.They need to chill. Baked gooey bars need to cool and then chill before you cut them. If you don't allow them to get cold and set, the gooey part will ooze out and you'll have a big mess and not pretty bars.
Nutritional information given is an automatic calculation and can vary based on the exact products you use and any changes you make to the recipe. If these numbers are very important to you, I would recommend calculating them yourself.