Line 2 large baking sheets with nonstick baking paper. Place the butter, icing sugar and vanilla bean paste or extract in a bowl and beat together with a hand-held electric whisk until light and fluffy. Sift in the flour and cornflour and stir until smooth, adding just enough of the milk to form a dough of piping consistency.
Place half of the mixture in a second bowl, sift in the cocoa powder and fold in until combined. Stir the coffee essence into the first bowl until evenly combined.
Spoon the cocoa mixture into one side of a piping bag fitted with a star-shaped nozzle, and the coffee-flavoured mixture into the other side so that both mixtures will be piped out at the same time. Pipe about 20 whirls or other shapes on to the prepared baking sheets.
Bake in a preheated oven, 200°C (400°F), Gas Mark 6, for 10–15 minutes until lightly golden. Transfer to wire racks to cool. Serve dusted with chocolate powder.