Preheat oven to 350 degrees F (175 degrees C). Grease and line two 9-inch round cake pans with parchment paper. Then grease and flour the paper.
Sift together the cornstarch, flour, baking powder and salt.
Add the oil, milk, water and vanilla. Beat until it forms a very smooth batter.
In a separate bowl beat the egg whites until frothy, add the cream of tartar and beat until stiff peaks form. Gradually add the sugar and beat until very well blended.
Fold the egg whites into the batter. Pour batter into the prepared pans.
Bake at 350 degrees F (175 degrees C) for 35 to 40 minutes. Let cool then frost with Orange Frosting.
Cream the butter until light and fluffy. Add the orange zest and salt. Beat in the confectioners' sugar alternately with the orange and lemon juices. Continue to beat until light and fluffy. Use to frost cooled cake.