Preheat oven to 350 °F. Line a 9x9 baking pan with foil an grease very well.
In a large bowl, beat butter until smooth. Mix 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. Scatter 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 the edges pull away from the sides and the top is set. Be sure to cool bars completely in the pan before cutting into them.
Notes
Storage instructions:Triple Lemon Gooey Bars will stay fresh and soft for up to 5 days stored in an airtight container at room temperature.