Preheat the oven to 180ºC/Gas Mark 4.
Grind the coffee powder with the back of a spoon in a bowl, so that it is nice and fine.
In a stand mixer with a paddle attachment, place the butter and sugar into the bowl and cream until fluffy.
In a separate bowl, sift the plain flour, cornflour, cocoa powder and instant espresso powder. Add the dry ingredients to the mixer with the butter and sugar and mix until a dough is formed. Wrap in clingfilm and chill for 30 minutes.
Lightly dust the worktop with flour and roll out the dough to about 6mm (¼in) thick. Cut into circles with a 6cm (2¼in) round cutter and place onto 2 baking sheets lined with baking parchment. You only need to leave a little space between the biscuits as they do not spread too much.
Gather up the offcuts, reroll and continue until you have 30 biscuits. Chill for 15 minutes in the fridge.
Bake for 15-20 minutes. Cool on the trays for 5 minutes, then move to a wire rack to cool completely.