Preheat the oven to 180°C (350°F/Gas 4). Line a 12-hole shallow patty pan or mini muffin tin with paper cases.
Put the butter, sugar, flour, milk, vanilla and eggs in a bowl. Using electric beaters, beat on low speed for 2 minutes, or until well mixed. Increase the speed and beat for 2 minutes, or until pale and smooth.
Divide the mixture evenly among the cases and bake for 20 minutes, or until cooked and golden. Transfer to a wire rack to cool completely.
Whip the cream to soft peaks. Using a small sharp knife, cut shallow rounds from the top of each cake, then cut these in half. Spoon ½tablespoon of the cream into the cavity in each cake, then top with 1teaspoon of jam. Position two halves of the cake tops in the jam in each cake to resemble butterfly wings. Dust the cakes with icing sugar before serving.