Melt the butter and leave to cool a little.

Toast the hazelnuts in a dry frying pan for 4 - 5 minutes, stirring often. Coarsely chop them. I cheated and used my mini-blender for this, rather than do it by hand.

Whisk the sugar and eggs together until the mixture is smooth.

Add the cocoa and melted butter.  Mix in.

Break up the biscuits into small pieces.

Add the hazelnuts and biscuits.  Mix until everything is well-coated in the chocolate mixture.

Pour the mixture out onto a piece of baking paper and form in into a long log.

Wrap the log firmly in the paper and put it in the fridge for at least 4 hours to harden.

To make it prettier, you can sift icing sugar over it before you slice it.