Preheat oven to 400°F. Spray cookie sheets with vegetable pan spray. In large bowl, beat egg whites, sugar and flour until blended and smooth. Beat in melted butter, vanilla and salt. Preparing 2 cookies at a time, drop 2 tablespoons batter onto cookie sheet, about 6 in. apart. Spread with tapered spatula to form a 4 x 6 in. oval. Bake 6-8 minutes or until edges are golden.