Because Key limes aren’t commonly available, look for the bottled juice in supermarkets. Or swap in regular lime or even lemon juice—the flavor will be different but still delicious.
- 1. Preheat oven to 350°F. Line an 8-inch square baking pan with foil, leaving an overhang. Mist foil with cooking spray.
- 2. Make crust: Using an electric mixer on medium-high speed, beat butter and sugar until light, about 2 minutes. Beat in flour and salt. Press evenly over bottom of pan. Bake until firm and light golden around edges, 20 to 25 minutes. Cool slightly.
- 3. Make filling: Using an electric mixer on medium speed, beat eggs, yolk and sugar until smooth. Stir in lime juice and zest. Fold in flour. Pour filling over crust and bake until set, 20 to 30 minutes. Cool on a wire rack. Cover and refrigerate for at least 1 hour or overnight. Use foil overhang to remove bars from pan to cut. Dust with confectioners' sugar just before serving, if desired.