Line a 12-section bun tray with paper cake cases. Cut the marshmallows into pieces using kitchen scissors.
Put all the remaining cake ingredients in a bowl and beat with a hand-held electric whisk for about a minute until light and creamy. Stir in the marshmallow pieces and then divide the cake mixture between the paper cases.
Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 20 minutes or until risen and just firm to the touch. Transfer to a wire rack to cool.
Cut the marshmallows for the topping into small pieces and put about a third in a small saucepan with half the cream. Heat very gently until melted. Tip into a bowl and leave to cool.
Whip the remaining cream in a bowl until just holding its shape. Stir in the marshmallow cream and remaining marshmallow pieces, then pile on to the cakes. Scatter with extra chopped marshmallow pieces to serve.