Preheat oven to 350 degrees F. Line a 9x9 baking pan with parchment paper.
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.
Break up the remaining dough and crumble it over the top of the bars. Press lightly to seal.
Bake for about 29-32 minutes, until slightly browned. Cool completely.
Slice and enjoy!
These can be stored in an airtight container for up to one week.
To freeze: Let cool completely. Then transfer bars to a gallon size resealable bag in a single layer. Place a paper towel on top (inside the bag.) Seal and freeze. They will maintain best quality for up to a month. To defrost, let them sit on the counter to reach room temperature (don’t remove the paper towel, which absorbs the moisture as they thaw.)