Crush the biscuits and place in a bowl with the chopped nuts.
Melt the butter, in the microwave or in a saucepan, and mix with the nuts and crumbs.
Put the ring moulds on some parchment in a dish and divide the mixture between them, pressing down well.
Put them in the fridge to set.
Whisk the egg yolks and sugar together in a bowl.
Melt the chocolate in a bain marie by putting the pieces in a heatproof bowl and setting the bowl over a simmering pan of water.
Put 125 ml of the cream and milk in a saucepan and bring to a simmer.
Whisk the milk slowly into the egg and sugar mixture then return the mixture to the saucepan.
Cook over a medium heat, while stirring. The custard will be ready when the mixture coats the back of the wooden spoon.
Let the custard cool for 5 -10 minutes then mix with the melted chocolate.
Whip the remaining 75 ml of cream to soft peaks.
Add the whipped cream to the chocolate custard.
Fold the cream into the chocolate custard with a metal spoon.
Divide the mixture between the ring moulds.
Leave to set for 6 hours or overnight.
Set a mould on a serving plate and use a blowtorch to quickly heat around the outside edge. The mould should slip off.
Refrigerate until you are ready to serve.