Line a 12-section muffin tray with paper muffin cases. Put the lightly salted butter, caster sugar, flour, eggs and ground almonds or hazelnuts in a bowl and beat with a hand-held electric whisk for 1–2 minutes until light and creamy.
Reserve a handful of the chopped unblanched hazelnuts for decoration. Add the remainder to the cake mixture with the white and milk chocolate, mix together and then divide 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.
Beat together the unsalted butter, sugars and lemon juice in a bowl until pale and fluffy. Spread the frosting over the cakes with a small palette knife and decorate with the reserved nuts.