Preheat the oven to 180 degrees Celsius. Put the soft butter into a mixing bowl and beat with an electric mixer or wooden spoon for around 1 minute, and make sure that it is very creamy.
Whilst still beating, gradually add the sugar into the mixture and continue beating for around 3-4 minutes. The mixture will be almost white and very fluffy. Break the eggs in a small bowl, and beat with the vanilla extract but not too much. Slowly add this mixture a tablespoonful at a time, and beat well after each addition. If it looks as though it may curdle, add a tablespoon of the sifted flour and continue.
Sift the flour again, but into the mixture. Then, carefully fold the flour into the mixture using a large metal spoon. Make sure this is done lightly because as much air as possible is needed.
When you can see no more flour in the mixture, divide it between two tins. Spread the mixture all around the tin, right into the edges. Bake for 20-25 minutes, and make sure it springs back when pressed.
After leaving the cakes to cool for around one minute, take the cakes out of the tins and leave to cool completely before adding jam. Set both sponges upside down, spreading jam over one and butter icing over the other before fixing it together and dusting icing sugar on top.