Preheat the oven: Preheat to 180°C (350°F) or 160°C (320°F) if using a convection oven. Grease two 20cm (8-inch) round sandwich tins and line the bases with nonstick baking paper.
Prepare the batter: In a large mixing bowl, combine the margarine, sugar, eggs, flour, and baking powder. Beat with an electric mixer until the mixture is well blended and smooth.
Divide and bake: Divide the mixture evenly between the prepared tins. Level the tops, then bake in the preheated oven for 25–30 minutes, or until the sponges are well risen and the tops spring back when lightly pressed with a finger.
Cool the sponges: Allow the cakes to cool in their tins for a few minutes, then turn them out, remove the baking paper, and let them cool completely on a wire rack.
Prepare the filling: Measure the cream into a large bowl and whisk until stiff peaks form.
Assemble the cake: Once the cakes are completely cool, spread the jam over the bottom of one sponge and top with the whipped cream. Place the second sponge on top. Transfer the cake to a serving plate and sprinkle with a little extra superfine sugar.
