Preheat the oven to 170C fan (180C conventional). Grease and line two 20cm round cake tins.
Beat the butter and caster sugar together until pale and fluffy, about 4 minutes.
Beat in the eggs one at a time, adding a spoonful of flour with each if the mixture curdles.
Sift in the self-raising flour and baking powder, then fold gently with a spatula until just combined.
Stir in the milk and vanilla extract for a soft dropping consistency.
Divide the batter between the tins, level the tops, and bake for 20 to 25 minutes until golden and springy.
Turn out onto a wire rack and leave to cool completely.
Spread jam over one sponge and whipped cream over the other, sandwich together, and dust with icing sugar.