Heat oven to 180C/fan 160C/gas 4. Melt 85g chocolate in the microwave on High for 1 min or over a pan of simmering water.
Beat in the butter, sugars, vanilla and egg until smooth, then stir in three-quarters of both the nuts and remaining chocolate, then the flour and bicarbonate of soda.
Heap 12 spoonfuls, spaced apart, on 2 baking sheets (don’t spread the mixture), then poke in the reserved nuts and chocolate. Bake for 12 mins until firm, then leave to cool on the sheets. Can be stored in a tin for up to 3 days.