Cream the butter, sugar, vanilla, and salt until fluffy. Beat in the egg whites just enough to blend them, and then the flour only until mixed -- do not overbeat. Butter and flour a baking sheet. Using a pastry bag with a plain round 3/8-inch opening, pipe out tongues about 1/2 inch apart. Hold the tip almost vertical to the baking sheet and very close to it so that you get a very thin layer of batter. Bake at 425 for 4 to 5 minutes, or until the cookies have browned lightly around the edges but are paler in the center. Remove them immediately to a cooling rack. If they harden before you can take them off the sheet, return the pan to the oven briefly to soften them again. Store in an airtight container.