Beat butter and icing sugar in a bowl with an electric mixer until light and creamy. Fold in combined sifted cornflour, plain flour and cocoa.
Roll 2 teaspoons of mixture into balls. Place about 2 cm apart on oven trays lined with baking paper. Flatten slightly with the back of a spoon.
Cook biscuits in a moderate oven (180°C) for about 8 minutes, or until lightly browned. Stand biscuits on trays for 5 minutes, before transferring to a wire rack to allow to cool.
Place chocolate in a medium heatproof bowl over a small pan of simmering water. Stir over heat until chocolate is smooth. Place a biscuit on a fork and dip into chocolate mixture to coat all over. Shake off any excess chocolate. Place on a tray lined with baking paper. Repeat with remaining biscuits and chocolate. Refrigerate the biscuits until set.
Meanwhile, to make mocha cream, beat butter in a small bowl with an electric mixer until light and fluffy. Gradually beat in icing sugar until combined. Dissolve the coffee in hot water and add to butter mixture, beating until combined.
Spoon cream into a piping bag fitted with a 1.5 cm fluted nozzle. Pipe small rosettes on each biscuit. Refrigerate until ready to serve.