Use a 6-in. sandwich cake-tin or a funnel tin which is not greased. Sift the flour and sugar separately three times, then sift the flour with ¼ of the sugar. Put the egg white and salt in a large, clean, dry bowl and whisk until frothy. Sprinkle on the cream of tartar and continue whisking till the white stands up in peaks. Avoid over whisking so that the white has lost its glossiness. Lightly beat in the sugar and flavouring, then, using a tablespoon, fold in the sifted flour and sugar carefully and gradually. Pour into tin and gently cut through mixture with a knife to release air bubbles. Bake for 40–45 min. in a very cool oven (290° F., Gas 1), increasing the heat to 335° F., Gas 3) for the last 10–15 min. Allow the cake to stand in the inverted tin for 30 min., then turn out on to a cooling tray.