In another bowl, with the same beaters, cream the butter with the sugar and vanilla until the mixture is light and fluffy. Add the egg yolks and beat until smooth. At low speed, add the dry ingredients, then half the egg whites. With a spatula, gently fold in the remaining egg whites. Pour into the prepared pan, and spread evenly. Bake for about 55 minutes or until a toothpick inserted in the centre of the cake comes out clean.
