Beat 1-1/4 cups granulated sugar and spread until creamy. Add yogurt, egg whites, lemon peel and vanilla; beat well. Gradually add combined oats, flour, baking powder and baking soda; mix well. Cover; chill 1 to 3 hours. Heat oven to 375°F. Lightly spray cookie sheets with nonstick cooking spray. With lightly floured hands, shape dough into 1-inch balls. Place 3 inches apart on cookie sheets. Flatten to 1/8-inch thickness with bottom of glass dipped in granulated sugar. Bake 10 to 12 minutes or until edges are lightly browned. Cool 2 minutes on cookie sheets; remove to wire rack. Sift powdered sugar over warm cookies. Cool completely. Store tightly covered.