Preheat the oven to 160C.
Chop 80g of the chocolate into rough,irregular chunks.
Break the remaining chocolate into a large heatproof mixing bowl and melt over a pan of simmering water,making sure the bowl doesn't touch the water.
Stir the chocolate until melted,then add the sugar,butter,peanut butter,egg and vanilla and beat with a wooden spoon until well mixed.
Stir in the sifted flour,then the nuts and mix well. Let it sit for 20 minutes covered on the bench.
Roll into 12 balls,about a large tablespoon each,and place on two lined baking sheets,leaving room for them to spread (you may need to bake them in two batches).
Stick the remaining chocolate chunks and marshmallow pieces (2-3 in each) into the cookies. Try to keep them in the centre so they don't melt over the edge of the cookie.
Bake for 10 minutes until they are very slightly darker around the edges.
Soft or crunchy? The smell will let you know they are ready. They will be soft in the middle,but will crisp up as they cool. Cook them a little longer for crisper cookies. Let them cool and firm up for a few minutes on the baking sheet;they will break if you move them while still hot.
Once cool,lift with a wide spatula onto a cooling rack. They will stay crisp in an airtight tin for 3-4 days.