MAKE CUPCAKES: Preheat oven to 350°F. Line 12 muffin cups with paper liners.
SIFT together flour, salt and baking powder in a medium bowl; set aside. In a large bowl, beat together butter, sugar, lemon zest and lime zest with a mixer at medium speed until light and fluffy, 3 to 4 minutes, scraping down sides of bowl occasionally. Beat in eggs, one at a time. Gradually add flour mixture to butter mixture alternatively with soda, beginning and ending with flour mixture, beating just until combined after each addition. Divide batter evenly among prepared muffin cups.
BAKE for 18 to 23 minutes or until a wooden pick inserted in center comes out clean. Cool for 10 minutes in pan, then remove from pan and cool completely on a wire rack.
MAKE FROSTING: Beat together confectioners’ sugar, butter, lime zest and salt in a large bowl at medium speed, just until combined.
ADD lemon juice gradually and beat until frosting is light, fluffy and has a smooth, pipeable consistency. Transfer to a piping bag fitted with a medium open star pastry tip (Wilton 1M) and pipe frosting onto cupcakes. Garnish with lemon and lime slices, if desired.