Preheat oven to 180 degrees C (350 degrees F). Grease a 26cm (10 inch) tube tray.
Beat egg whites and cream of tartar until soft peaks form. Add 3/4 cup sugar gradually, beating until very stiff.
In a large bowl, cream butter or margarine and 1 cup sugar well. Add egg yolks, and beat until thick and fluffy. Sift flour, baking powder, and salt over batter. Fold in. Gently fold in egg whites. Pour batter into prepared tin.
Bake for 50 to 60 minutes, until an inserted wooden pick comes out clean. Cool. Ice with your favourite frosting.