Gradually beat in the sugar, then keep on beating for 3 to 4 minutes or until the mixture turns almost white and becomes very fluffy in texture; scrape down the bowl from time to time. Break the eggs into a small bowl, add the vanilla and beat lightly with a fork just to break them up. Slowly add to the creamed mixture, a tablespoonful at a time, giving the mixture a good beating after each addition and frequently scraping down the bowl. This will take about 5 minutes. If the mixture looks as if it is about to curdle, add a tablespoon of the sifted flour and then continue adding the last portions of egg.