Preheat oven to 350 degrees.
Line a 9x9 inch baking pan with foil. Spray with cooking spray and set aside.
Beat butter with a mixer until soft and fluffy.
Add in cake mix,egg, lime juice and zest and continue beating until combined.
Press about two-thirds of the cake mixture onto the bottom of the foil-lined baking pan.
Sprinkle white chocolate chips over the top.
Drizzle the sweetened condensed milk over the top of the chocolate.
Crumble the remaining cake mixture over the top of the sweetened condensed milk, trying to spread it out as much as possible. Press lightly.
Bake for 29-32 minutes, or until edges start to brown.
Let bars cool completely before cutting.
Top with additional lime and lemon zest if desired.
Store in an airtight container for up to a week.