In a medium bowl, combine flour, salt and baking soda. Sift. Set aside. In the bowl of an electric mixer with a paddle attachment, cream butter and sugar until light and fluffy; 3-4 minutes. Add egg, lime juice and zest, mixing until combined. Turn mixing speed on low adding flour slowly until all is incorporated. Add corn meal until fully mixed. Make cookies into ping-pong sized balls (or 32 grams). Place on a tray and refrigerate until very cold. When chilled, place cookie balls (12 at a time) on the whoopie pie pan and bake for 12 minutes in a 350 degree oven. (You can use a regular baking sheet covered with parchment or a silicone liner but the cookies will not look as you see them here. They will be flat.) Remove the cookies from the oven and let them finish cooking on the tray. The edges will be slightly brown and the middles very soft. Do not disturb them for at least 20 minutes ( It makes it easier if you have two whoopie pies to work with. Never put cookie dough onto a hot pan.) Remove to a tray until completely cool.When cookies are completely cool, mix together confectioners' sugar, lime juice and zest until fully combined. If it is too runny add more sugar to get a pourable but thick consistency. Frost each cookie. Let dry completely or overnight before stacking.