Pre-heat your oven to 170°c (150°c for fan-assisted oven, Gas Mark 4 or 325°F). Grease and line two 18cm (7 inch) sandwich tins with greaseproof paper. Set aside.
In a large bowl, cream together the margarine and sugar until light and fluffy.
Add the eggs one at time, ensuring each is mixed through before adding the next.
Sift in the flour and gently fold to create a batter.
Transfer the batter to your pre-lined tins and bake in your pre-heated oven for 20-30 minutes until risen, golden and a skewer inserted comes out clean.
Leave to cool in the tins for a few minutes before transferring to a wire rack to cool completely
Once cool, sandwich the sponges with jam (& buttercream if desired). Top with a generous sprinkling of caster sugar.