Line a 12-section bun tray with paper cake cases. Blend the coffee powder with the boiling water. Put the butter, caster sugar, eggs, flour and baking powder in a bowl. Add the coffee and beat with a hand-held electric whisk for about a minute until light and creamy. Stir in the chopped walnuts. Divide the cake mixture between the paper cases.
Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 20 minutes or until risen and just firm to the touch. Transfer to a wire rack to cool.
Blend the coffee powder with the boiling water for the buttercream. Beat with the butter and icing sugar in a bowl until pale and creamy.
Spread the buttercream over the cakes using a small palette knife. Alternatively, put in a piping bag fitted with a star nozzle and pipe swirls over the tops of the cakes. Decorate each with a walnut half.