Grapefruit Cake

1 cup flour
3/4 cup sugar
1 1/2 teaspoons baking powder
1/4 teaspoon salt
1/4 cup peanut oil or canola oil
1 1/2 teaspoons grated grapefruit zest
1/3 cup grapefruit juice
3 egg yolks
4 egg whites
1/4 teaspoon cream of tartar

Grapefruit Frosting
2 teaspoons grated grapefruit zest
1-2 teaspoon grapefruit juice
1/2 teaspoon vanilla extract
1 cup sifted powdered sugar, more as needed for desired consistency

In a medium bowl stir together flour, sugar, baking powder, and salt.
Make a well in the center.
Add oil, grapefruit zest, juice, and egg yolks.
Beat until smooth with an electric mixer.
Wash beaters thoroughly with soap and hot water to remove all oil!
In a large bowl, whip egg whites until frothy, add cream of tartar, and beat until very stiff peaks form.
Gradually pour flour mixture into egg whites, folding in gently.
Pour into a ungreased 9"x5"x3" loaf pan.
Bake in a 350 degree F for 25-30 minutes, until it tests done with a toothpick.
Invert over a wire rack, and cool in pan.
Combine all frosting ingredients, and spread over top of cake.