Preheat oven to 180 degrees C. Butter 3 round 20cm cake tins and line the bottoms with baking paper.
In a bowl, sift together the flour, salt and baking powder three times to well combine.
In a separate bowl, beat the egg whites until foamy. Add 100g of the sugar and beat to soft peak stage.
In a third bowl, cream the softened butter until smooth. Gradually add remaining sugar and cream until fluffy. Add the flour alternately with the milk to the butter in small batches, beating until smooth after each addition.
Mix in the vanilla then gently fold in the egg whites until no streaks remain. Divide the cake mix evenly between the cake tins and smooth down.
Bake 25-30 minutes or until a skewer comes out clean. Cool 10 minutes in the tin then turn out onto racks to cool completely before icing.