Preheat oven to 350°.
Coat an 8 x 4-inch loaf pan with cooking spray; dust with 1 teaspoon flour. Set aside.
Beat granulated sugar and butter at medium speed of a mixer until well-blended (about 4 minutes). Add egg whites and egg, 1 at a time, beating well after each addition. Beat in lemon rind and vanilla. Lightly spoon 1 2/3 cups flour into dry measuring cups; level with a knife. Combine 1 2/3 cups flour and next 4 ingredients (flour through salt), stirring well with a whisk. Add flour mixture to sugar mixture alternately with buttermilk, beginning and ending with flour mixture. Pour batter into prepared pan; bake at 350° for 1 hour or until a wooden pick inserted in center comes out clean. Cool in pan 10 minutes on a wire rack; remove from pan. Poke holes in top of cake using a skewer. Combine powdered sugar and lemon juice in a small bowl; brush over warm cake. Cool completely.
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