1Preheat oven to low, 150 C. Lightly grease and line two oven trays with baking paper.
2In a small jug, combine coffee and water, stirring to dissolve. Or simply extract Nespresso coffee.
3In a bowl, using an electric mixer, beat butter and icing sugar together, until creamy. Beat in coffee mixture. Lightly fold in combined sifted flours and custard powder.
4Roll tablespoons of mixture into balls and arrange on trays 3 cm apart (see tip). Flatten slightly and push a walnut into half the biscuits. Bake 20 to 25 mins. Allow to cool on trays.
5Meanwhile, make the filling in a jug; dissolve coffee in water and set aside. In a small bowl, using an electric mixer, beat butter until pale. Gradually add icing sugar, beating until smooth. Beat in coffee. Spread over flat side of plain biscuits. Sandwich together with remaining biscuits, walnut side up.
Photo: Bauer/Rob Shaw