Preheat the oven to 180°C (350°F). Grease and line two 20cm sandwich tins with parchment paper.
Beat the caster sugar and softened butter together until pale and creamy.
Add the eggs one at a time, beating well after each addition. If the mixture starts to curdle, add a tablespoon of the flour to stabilise it.
Sift the self-raising flour and baking powder into the mixture and fold in gently. Add the milk and fold until the batter is smooth.
Divide the batter evenly between the two prepared tins and smooth the tops. Bake for 20-25 minutes, or until golden brown and a skewer inserted into the centre comes out clean.
Let the cakes cool in their tins for a few minutes, then turn out onto a wire rack to cool completely.
Spread the jam generously over one sponge, then spread the whipped cream over the jam. Place the second sponge on top and press down gently. Finish with a dusting of caster or icing sugar.