Lightly grease two baking trays.
Rub the butter or margarine into the flour until the mixture resembles fine breadcrumbs.
Stir in the sugar and ground walnuts, then add the egg and mix well to make a firm dough.
Knead gently, then roll out the dough on a lightly floured board to a thickness of 5 mm/1/4 in.
Cut into rounds with a 5 cm/ 2 in plain cutter and re roll the trimmings to produce as many biscuits as possible.
Using a palette knife, carefully transfer the biscuits to the prepared baking trays.
Chill for 30 minutes, or longer if you have time.
Bake in a moderately hot oven (190C, 375F, gas 5) for 15 minutes, or until very lightly browned.
Remove the hot biscuits from the trays and transfer them to a wire rack to cool completely.
While the biscuits are cooling, make the butter and glace icings according to the recipe instructions.
Sandwich the biscuits together in pairs with the coffee butter icing and ice the tops of the biscuits with coffee glace icing.
To decorate, press a halved walnut in the centre of each biscuit and leave to set.