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Preheat oven to 180°C/ fan 160°C/gas 4. Dissolve the coffee granules in the boiling water. Line a 12-hole cupcake tin with paper cupcake cases.
Place the butter, sugar, eggs, flour and cooled coffee mixture in the bowl fitted with the kneading/grinding tool. Process on speed 9 for 1 min. Add the chopped walnuts and mix at speed 3 for 15 s.
Spoon mixture into cases. Bake for 20 to 25 mins or until they spring back when lightly pressed. Cool on a wire rack.
For the buttercream icing, dissolve the coffee granules in the boiling water. Place butter, icing sugar and cooled coffee mixture in clean bowl fitted with kneading/grinding tool. Mix at speed 3 for 30 s and then speed 9 for 1 min.
Pipe a swirl of buttercream icing on top of cakes and decorate with a walnut.