Preheat oven to 350 degrees F (175 degrees C). Grease and flour one 9 inch tube pan.
In a medium bowl, whisk together the flour, 3/4 cup sugar, baking powder, and salt. Make a well in the center and add the oil, egg whites, lemon juice, and lemon zest. Beat until smooth.
In a separate medium bowl, beat the egg whites and cream of tartar until soft peaks form. Gradually add the 1/4 cup sugar, beating until stiff peaks form. Gently fold the egg whites into the lemon batter.
Pour the batter into the prepared pan. Bake for 25 to 30 minutes, or until a toothpick inserted into the center of the cake comes out clean.
Allow the cake to cool in the pan for 10 minutes, then turn out onto a wire rack to cool completely.
To make the icing, beat the butter until creamy. Beat in the confectioners' sugar, then the lemon juice and zest. Beat until light and fluffy.
Spread the icing over the cooled cake.
Interesting Facts
Chiffon cake was invented in the 1920s.
Chiffon cakes are incredibly light and fluffy.
Chiffon cakes get their light texture from beaten egg whites.