Preheat the oven to 180 C / Gas 4. Grease three 20cm (8 in) round cake tins and line the bottoms with baking parchment.
Sift the flour, baking powder and salt together three times. Set aside.
In a separate bowl, beat egg whites until foamy. Add 100g of the sugar gradually, and continue beating only till soft peaks form.
Beat butter until smooth. Gradually add remaining sugar, and cream together until fluffy. Add sifted ingredients alternately with milk a small amount at a time, beating after each addition until smooth. Mix in vanilla. Add egg whites, and fold in gently till no streaks of egg white remain. Divide the cake mixture evenly between the three prepared tins.
Bake in the preheated oven for 25 to 30 minutes, or till a skewer inserted into the centre comes out clean. Cool slightly in tin, then remove to cool completely on wire racks before icing and decorating.
Sifting ensures a lighter texture, so don't skip this step!