Preheat oven to 190C. Cream the butter until light. Gradually add the sugar. Beat until pale in colour, add egg yolk and dissolved coffee and mix in well. Stir sifted flour and cornflour into mixture to form soft dough. Roll into small balls, press flat on a greased baking tray with the bottom of a glass. Bake for 10 – 15 minutes until golden brown. When cool place a teaspoon of marshmallow fluff on the biscuit and dip the whole biscuit into the melted chocolate. Allow to set on grease proof paper at room temperature.