Cream together the butter and sugar,
then mix in the egg yolks. Stir in the flour and baking powder
alternately with the milk. Stir in the lemon and vanilla essences.
Whisk the egg whites until they form soft peaks, then gently fold into
the mixture. Turn into a greased 900 g / 2 lb loaf tin (pan) and bake
in a preheated oven at 150°C/300°F/gas mark 2 for 1 1/4 hours until
golden brown and springy to the touch.