Step 1 Preheat oven to 325-degrees. Sift together cake flour, baking powder and salt in medium bowl. Stir in sweetener; set asked.
Step 2 In large bowl, combine oil, egg yolks, buttermilk, lemon juice, lemon rind and vanilla; beat with electric mixer until smooth. Incorporate flour mixture on low speed. Beat on medium speed 30 seconds or until smooth.
Step 3 In separate bowl, with clean beaters, beat egg whites and cream of tartar until foamy. Gradually add sugar and beat whites until they are stiff, but not dry.
Step 4 Gently stir in 1/4 of the whites to loosen batter; gently fold in remainder of whites. Spoon batter into ungreased 10-inch tube pan; spreading evenly.
Step 5 Bake 30 minutes or until cake springs back when gently touched. Cool upside down by placing pan on bottle or funnel for 45 minutes. Sift powdered sugar over cake before serving, if desired.