Preheat oven to 325°., Sift flour, sugar, baking powder and salt together twice; place in another large bowl. In a small bowl, whisk egg yolks, water, oil, lemon zest and vanilla until smooth. Add to flour mixture; beat until well blended., Add cream of tartar to egg whites; with clean beaters, beat on medium speed just until stiff but not dry. Fold a fourth of the whites into batter, then fold in remaining whites. , Gently spoon batter into an ungreased 10-in. tube pan. Cut through batter with a knife to remove air pockets. Bake on lowest oven rack 50-55 minutes or until top springs back when lightly touched. Immediately invert pan; cool completely in pan, about 1 hour., Run a knife around sides and center tube of pan. Remove cake to a serving plate., In a small bowl, combine frosting ingredients; beat until smooth. Spread over cake.