Break both sorts of chocolate into small chunks and place in a glass bowl along with the butter. Place the bowl over a pan of boiling water and stir gently until melted then remove from the heat. Break the biscuits into small chunks placing in a freezer bag and bashing with a rolling pin if needed. Add the biscuits, fruit, nuts and marshmallows into the bowl and stir gently ensuring everything is coated in the melted chocolate. If using brazil nuts and large marshmallows roughly chop before adding to the bowl.

Using either a foil tray or rectangular baking dish lined with baking paper, tip in the mixture and press down well although the top will remain bumpy. Cover and place in the fridge for at least an hour until set. Turn out the whole slab onto a chopping board and dust liberally with icing sugar. Slice into chunks by first cutting into 4-6 strips lengthways depending on size and then cutting across the strips to create cubes.

Cut into roughly 1 inch cubes to serve for afternoon tea or smaller chunks if using as petit fours.