Put the butter and sugar in a saucepan and heat gently until the butter has melted and the sugar dissolved. Remove the pan from the heat and stir in all the remaining ingredients except the chocolate.
Spoon tablespoons of the mixture, well spaced apart, on to 3 baking sheets lined with nonstick baking paper. Flatten the mounds slightly. Cook one baking sheet at a time in the centre of a preheated oven, 180°C (350°F), Gas Mark 4, for 5–7 minutes until the nuts are golden.
After removing each baking sheet from the oven, neaten and shape the edges of the cooked biscuits by placing a slightly larger plain round biscuit cutter over the top and rotating to smooth and tidy up the edges. Leave to cool.
Melt the chocolate in a heatproof bowl set over a saucepan of gently simmering water. Peel the biscuits off the lining paper and arrange upside down on a wire rack. Spoon the melted chocolate over the flat underside of the biscuits and spread the surfaces level. Leave to cool and harden.