1. Preheat oven to low, 150c. Lightly grease and line two oven trays with baking paper.
2. In a small jug, combine Lavazza Prontissimo Classico Instant Coffee and water, stir to dissolve.
3. In a bowl, using an electric mixer, beat Planta Margarine and icing sugar together, until creamy.
4. Beat in coffee mixture. Lightly fold in combined sifted flours and custard powder.
5. Roll tablespoons of mixture into balls and arrange on trays 3 cm apart. Flatten slightly and push a walnut into half the biscuits.
6. Bake 20 to 25 mins. Allow to cool on trays.
7. Meanwhile, make the filling in a jug; dissolve Lavazza Prontissimo Classico Instant Coffee in water and set aside.
8. In a small bowl, using an electric mixer, beat butter until pale.
9. Gradually add icing sugar, beat until smooth.
10. Add in Lavazza Prontissimo Classico Instant Coffee and spread over flat side of plain biscuits.
11. Sandwich together with remaining biscuits, walnut side up.