Key lime bars

Preheat oven to 350 degrees. Spray a 9x13 inch baking pan with non-stick cooking spray. Reserve 1 tablespoon cake mix. Pour remaining cake mix, melted butter and egg yolk into a large bowl. Blend for 1-2 minutes with an electric mixer until well mixed and crumbly. Press mixture into the bottom of the prepared pan. Bake for 10 minutes. While crust bakes, whisk together 4 eggs, condensed milk, lime juice and reserved tablespoon of cake mix until smooth. Stir in lime zest. Spread the lime mixture over the warm crust. Return to oven and bake an additional 20 minutes or until topping is set. Cool completely and refrigerate until cold before serving.