Beat egg whites until soft peaks form. Gradually add, while continually beating, cream of tartar and 1/2 cup sugar. Beat until very stiff and glossy. Set aside.
Sift into large bowl flour, baking powder and salt and 1 cup sugar. Add oil, 1/2 cup milk and flavoring. Beat 1 minute at medium speed. Add 1/2 cup milk and egg yolks. Beat 1 minute, scraping bowl constantly.
Fold egg whites in gently, but thoroughly. Turn into greased and floured pans (two 8 inch or one 9 x 13 inch or cupcake). Bake at 350 degrees for about 30 minutes for layer pans - 45 minutes for sheet cake - less time for cupcakes.
