Step 1

Heat the butter, chocolate and golden syrup in a heavy-based saucepan over a gentle heat. Remove from the heat, scoop out about 125ml/4½fl oz of the melted mixture and set aside in a bowl.

Step 2

Place the biscuits into a plastic freezer bag and crush them with a rolling pin until some have turned to crumbs but there are still pieces of biscuit remaining.

Step 3

Fold the biscuit pieces and crumbs into the melted chocolate mixture in the saucepan, then add the marshmallows.

Step 4

Tip the mixture into a 24cm/9in square baking tin and smooth the top with a wet spatula.

Step 5

Pour over the reserved 125ml/4½fl oz of the melted chocolate mixture and smooth the top with a wet spatula.

Step 6

Refrigerate for about two hours or overnight.

Step 7

To serve, cut into 24 fingers and dust with icing sugar.