Preheat the oven to 375ºF.  Grease the cookie sheets. Cream together the butter and sugar in a medium-size bowl until light, then mix in the egg yolk. Stir in the lemon juice and zest, then fold in the flour and salt. Drop the batter by the teaspoonful onto the cookie sheets, allowing plenty of room for the cookies to spread. Bake until the edges of the cookies begin to brown, 8 to 10 minutes. Cool the cookies slightly, then loosen them from the sheet with a spatula to prevent sticking. Sift confectioners sugar on top and cool completely before serving. Store the cookies in airtight tins.