Mix together flour, cornflour, baking powder and 2 teaspoons salt and sift together three times. Beat egg whites until foamy, add 200g caster sugar gradually and continue beating only until mixture will stand in soft peaks. Cream margarine, add 500g caster sugar gradually and cream together until light and fluffy. Add flour, alternately with milk, a small amount at a time, beating after each addition until smooth. Add 2 teaspoons vanilla and egg whites and beat thoroughly into a thick creamy mixture.
Turn the cake mixture into one round 26cm tin and one round 20cm tin, 7.5cm deep, each lined on bottom with paper, filling each tin about half full of batter.
Bake in moderate oven 180 C / Gas 4, placing the smaller layer towards front of oven for quicker and easier removal when done. Bake the 20cm cake for 1 hour or until done; bake the 26cm cake about 10 minutes longer.
Cool cakes in tins on racks for 15 minutes. Then loosen from sides of tins with spatula, turn out, remove paper and turn right side up on racks to cool. When thoroughly cooled, at least 2 hours, brush cakes to remove any loose crumbs.
To make the icing, cream 225g butter; add part of the icing sugar gradually, blending after each addition. Mix in 4 teaspoons vanilla and 1/2 teaspoon salt. Then add remaining sugar alternately with milk, until of right consistency to spread. Beat after each addition until smooth.
Once the cakes are completely cool, place the larger of the two cakes on a serving plate as the bottom layer then spread the top with a little of the buttercream. Place the second smaller cake on top and cover the entire cake with the buttercream and serve.