First, melt the chopped dark chocolate with the butter and the golden syrup in a large bowl that is sitting over a saucepan of simmering water on a low heat.
Stir in the crushed biscuits and the chopped toasted hazelnuts or almonds, whichever you're using, if you're using them, until everything is well-blended. Spread into a 23cm (9in) diameter round cake tin or a 20cm (8in) square tin, lined with greaseproof or parchment paper.
Refrigerate the cake for a couple of hours until it is well set, or pop it into the freezer for 45 minutes.
Cut into 16 pieces to serve.