If you can't find key limes, you can use regular Persian limes, but the squares won't be quite as tart.Enjoy this tart dessert treat that's low in sodium. For variation, try this recipe with lemons.
Preheat oven to 350°.
Place first 5 ingredients in a medium bowl; beat with a mixer at medium speed until creamy (about 2 minutes). Lightly spoon 1 cup flour into a dry measuring cup; level with a knife. Gradually add 1 cup flour to butter mixture, beating at low speed until mixture resembles coarse meal. Gently press two-thirds of mixture (about 1 1/3 cups) into bottom of an 8-inch square baking pan coated with cooking spray; set remaining 2/3 cup flour mixture aside. Bake at 350° for 12 minutes or until just beginning to brown.
Combine 2/3 cup sugar, 3 tablespoons flour, baking powder, and 1/8 teaspoon salt in a medium bowl, stirring with a whisk. Add lime juice and eggs, stirring with a whisk until smooth. Pour mixture over crust. Bake at 350° for 12 minutes. Remove pan from oven (do not turn oven off); sprinkle remaining 2/3 cup flour mixture evenly over egg mixture. Bake an additional 8 to 10 minutes or until set. Remove from oven; cool in pan on a wire rack. Sprinkle evenly with powdered sugar.