Line 2 x 12-section bun tray with 18 paper cake cases. Put the butter, caster sugar, flour, eggs and vanilla extract in a bowl and beat with a hand-held electric whisk for 1–2 minutes until light and creamy. Stir in the chocolate chips. Divide the cake mixture evenly 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.
Pare curls from the piece of chocolate using a vegetable peeler – if the chocolate breaks off in small, brittle shards, try softening it in a microwave oven for a few seconds first, but take care not to overheat and melt it. Set the chocolate curls aside in a cool place while icing the cakes.
Spread the chocolate frosting all over the tops of the cakes using a small palette knife. Pile the chocolate curls on to the cakes and lightly dust with icing sugar.