Break gelatine into small pieces, combine in the bowl of an electric mixer with coffee and 50ml cold water and set aside. Combine sugar, corn syrup and 125ml water in a small saucepan, stir over medium-high heat until sugar dissolves, then bring to the boil and cook until syrup reaches 115C on a sugar thermometer (2-3 minutes).
Whisk gelatine mixture on low speed, add hot syrup in a thin steady stream down the side of the bowl, add vanilla and whisk on high speed until thick and fluffy and tripled in volume (8-10 minutes). Spoon into a lightly oiled 18cm x 23cm cake tin, quickly smooth top with a lightly oiled spatula and stand at room temperature until set (2-3 hours).
Sift icing sugar, cocoa and cornflour into a bowl. Turn marshmallow out of tin onto a lightly oiled piece of baking paper, cut into 3cm cubes with a lightly oiled knife, then toss in cocoa mixture. Store in an airtight container, in cocoa mixture, until required. Marshmallows will keep for a week.