Line a 12-section bun tray with paper cake cases. Put the pistachio nuts in a heatproof bowl, cover with boiling water and leave to stand for 30 seconds. Drain thoroughly and rub between several thicknesses of kitchen paper to loosen the skins. Peel away the skins – you needn't be too thorough about this, as long as most of the skins are removed. Process in a food processor until finely chopped.
Beat the butter, caster sugar, eggs, flour, baking powder and 50 g (2 oz) of the ground nuts with a hand-held electric whisk in a bowl for about a minute until light and creamy. Divide the 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.
Peel away the paper cases. Whip the cream with the icing sugar and liqueur in a bowl until only just holding its shape. Transfer half to a separate bowl and reserve. Spread the remainder around the cakes and roll in the remaining ground nuts to coat the sides. Spoon the reserved flavoured cream on top.