1. Heat oven to 350 degrees F. Lightly grease a 9 x 2-inch round cake pan. Line bottom of pan with waxed paper and lightly grease paper.
2. In a large bowl, sift together flour, sugar, baking powder and salt.
3. In a medium-size bowl, whisk together egg yolks, oil, water, grapefruit juice and lemon zest until smooth. Whisk in flour mixture.
4. Beat egg whites and cream of tartar with an electric mixer in a medium-size bowl until stiff peaks start to form. With a spatula, gently fold whites into yolk mixture, until just blended. Pour into prepared pan.
5. Bake at 350 degrees F for 25 to 30 minutes, or until cake springs back when touched lightly in the center. Invert cake in pan; cool completely on a wire rack.
6. Run a spatula around edge of cake. Carefully remove cake from pan. With a serrated knife, cut cake in half horizontally.
7. Spread 1 cup of the Cream Cheese Frosting on bottom half of cake. Top with several grapefruit sections. Cover with second layer of cake and frost top and sides. Serve cake garnished with remaining grapefruit sections.
1. With mixer, beat cream cheese at high speed until light. Add lemon juice and lemon zest. Gradually add powdered sugar and beat until well blended. Add a few drops of yellow food coloring, if desired.