Preheat oven to 350 degrees. Line a 9x9 baking pan with foil an grease very well.

In a large bowl, beat butter until smooth (with a stand or hand mixer). Stir in cake mix, egg, lemon zest and lemon juice until combined. Mixture will be a thick, dough-like consistency.

Press ⅔ of the mixture down into the bottom of the prepared pan. Sprinkle white chocolate chips on top and drizzle sweetened condensed milk on top of that in an even layer. Sprinkle the remaining dough on top, trying to fill as much space as you can. There will be holes and that is okay, the dough will spread a bit in the oven.

Bake for 25-30 minutes, until very lightly browned on top and edges pull away from the sides. Be sure to cool bars completely in the pan before cutting into them or else it will be a gooey mess.

Triple Lemon Gooey Bars will stay fresh and soft for up to 5 days stored in an airtight container at room temperature.