Heat oven to
375F (190C). Line the bottom of a 9-inch (23-cm) round layer cake pan with
waxed paper and grease sides of pan. Beat egg until thick and
lemon-colored in small mixer bowl (about 5 minutes at high speed on the
mixer). Beat in sugar gradually. Stir in water and vanilla. Sift flour,
baking powder and salt together into mixture and beat until smooth. Pour
into prepared pan. Bake about 12 minutes or until top springs back when
touched lightly. Turn out on rack and strip off waxed paper. (This should
be very thin layer of cake.) Cool.