1/2 cup firmly packed light brown sugar
2 1/2 cups all purpose-flour, divided
Line a 13 x 9-inch baking pan with heavy-duty aluminum foil.
Beat butter in large mixer bowl on high speed of mixer 30 seconds.
Add brown sugar and beat until combined.
Add 2 cups of the flour; beat until mixture is crumbly.
Stir 1 tablespoon of the lemon zest.
Spray bottom of pan with cooking spray.
Press dough evenly onto bottom of prepared pan.
Bake 20 minutes.
Combine eggs, remaining 1/2 cup flour, granulated sugar, lemon and lime juices, and baking powder in large mixer bowl.
Beat on medium speed 2 minutes.
Stir in remaining lemon zest, and lime zest.
Spray sides of pan with cooking spray.
Pour egg mixture evenly over crust. Return pan to oven.
Bake 20 to 25 minutes or until edges are golden brown and center is set.
Cool in pan on wire rack 1 hour.
Cover and refrigerate at least 2 hours.
Cut into 24 squares when ready to serve. Sprinkle with powdered sugar.
Refrigerate any remaining squares.
Let eggs come to room temperature before beating for better volume.
Garnish each serving with additional lemon and lime zest, if desired.