Mix the flour, baking powder and caster sugar in a bowl. Make a well in the middle and add the egg and vanilla extract, if using. Pour in a little of the milk, then beat the egg lightly until it is mixed with the milk and vanilla. Gradually stir in the flour mixture and pour in the remaining milk a little at a time. Before all the milk is added and while the mixture is quite thick, beat well to remove all the lumps, then gradually beat in the remaining milk to make a smooth batter.