Grease two 20cm tins and line the bases with baking parchment.
Preheat the oven to 180°C.
Beat butter, sugar, eggs, flour, and baking powder in a bowl for two minutes until blended.
Divide the mixture between the tins and level the surfaces.
Bake for 25 minutes until golden and the sponge springs back when pressed.
Cool in the tins for a few minutes before transferring to a wire rack.
Peel off the parchment and allow the cakes to cool completely.
Spread jam on the bottom layer and place the other cake on top.
Sprinkle the top with caster sugar to serve.