Preheat the oven to 180C/350F/Gas 4.
Gently heat the knob of butter in a pan and brush two 6in-7in/15cm-17.5cm cake tins with the melted butter. Line the bottom of the two cake tins with a circle of greaseproof paper.
In a large bowl, cream together the butter and sugar until pale and creamy, using an electric whisk or a wooden spoon. Beat well to get lots of air into the mixture (this should take a couple of minutes).
Beat in the eggs one at a time. Add a tablespoon of flour if the mixture curdles.
Fold in the flour using a large metal spoon. Be careful not to over-mix it.
Pour the mixture equally between the two cake tins and level off the top with a spatula. Make a slight dip in the centre with the tip of the spatula if you don't want them to be pointed in the middle.
Place in the oven and bake for about 20 minutes, or until the cakes spring back when pressed gently with a finger and are pale golden in colour.
Remove from the oven and take them out of the tins after about 5-10 minutes. Place them on a wire rack to cool completely (for about half an hour).
Spread the sponge with the jam and the whipped cream, then carefully sandwich together.
Dust with icing sugar and serve.