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Preheat the oven to 180C/gas 4/fan 160C.Gather together and weigh all the ingredients.
Chop 200g/7oz of the plain chocolate into rough, irregular chunks. Chop the milk chocolate in the same way, but keep separate.
Break the remaining plain chocolate into a large heatproof mixing bowl. Melt in the microwave on medium for about 11⁄2 minutes (or over a pan of simmering water).
Stir the chocolate until melted, then tip in the sugar, butter, peanut butter, egg and vanilla and beat with a wooden spoon until well mixed. Stir in the flour, all the milk chocolate chunks, the nuts (no need to chop) and half the plain chocolate chunks. The mixture will feel quite soft, and drop easily from the spoon if you shake it.
Drop big spoonfuls in 12 piles on to 2 or 3 baking sheets, leaving room for them to spread (you may need to bake in batches). Stick the remaining chunks into the cookies (2-3 pieces in each).
Bake for 10-12 minutes until they are tinged very slightly darker around the edges. The smell will let you know they are ready. They will be soft in the middle, but will crisp up as they cool. (Cook for longer and you’ll have crisper cookies.) Let them cool and firm up for a few minutes on the baking sheet (they’ll break if you move them while still hot), then lift off with a wide spatula on to a cooling rack. They will keep crisp in an airtight tin for 3-4 days.