Line a 12-section bun tray with paper cake cases. Put all the cake ingredients in a bowl and beat with a hand-held electric whisk for 1–2 minutes until light and creamy. 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.
Drizzle the cooled cupcakes with 2 tablespoons of the coffee-flavoured liqueur.
Put the remaining liqueur in a bowl with the cream and whip until the cream is thickened and only just holds its shape. Spread the cream over the tops of the cakes using a small palette knife, swirling it right to the edges.
Pare curls from the piece of chocolate using a vegetable peeler – if the chocolate breaks off in small, brittle shards, try softening it in the microwave for a few seconds first, but take care not to overheat and melt it. Scatter the chocolate curls over the cakes and dust with a little cocoa or drinking chocolate powder. Store the cakes in a cool place until ready to serve.