Grease and line the base of 2 x 7” loose based cake tins.
Preheat the oven to 180℃ fan/200℃/Gas Mark 6.
Put the butter and sugar into a large bowl and beat together until very light and fluffy. It is ready when you notice the colour has become lighter. (Use an electric hand whisk or stand mixer if you have one.)
Scrape down the sides and bottom of the bowl so no mixture is stuck. Break in the eggs and beat very well. (It will begin to look a little split and odd at this point, but have faith.)
Now add the flour and, using a spoon, mix this in thoroughly by hand. (Don’t use the electric whisk if you have been using one.) Be sure there are no pockets of flour left in the mixture, especially at the bottom of the bowl.
Share the mixture evenly between the two prepared tins. Smooth out and bake in the middle of the oven for 20-25 minutes or until a skewer comes out clean from the centre of the cake.
Leave to cool in the tin for 10 minutes, then turn out onto a rack to cool completely.
Sandwich together with the jam and sprinkle with the caster sugar.