Combine first 3 ingredients in a large bowl, stirring well. Combine milk, oil, vanilla, and grapefruit rind in a small bowl, stirring well. Add milk mixture to flour mixture, stirring well.
Beat egg whites at high speed of an electric mixer just until foamy. Add cream of tartar, and beat until stiff peaks form. Gently fold one-third of beaten egg white into batter; gently fold in remaining beaten egg white.
Pour batter evenly into paper-lined muffin pans, filling two-thirds full. Bake at 350° for 18 to 20 minutes or until a wooden pick inserted in center comes out clean. Remove cupcakes from pans immediately, and let cool completely on wire racks. Spread Grapefruit Icing evenly over cupcakes.