Method Prep:10min › Extra time:3hr chilling › Ready in:3hr10min
Place the butter, dark chocolate and syrup into a heatproof bowl and warm over a pan of simmering water, ensuring the bowl doesn't touch the water, for 5 to 10 minutes or until melted. Stir then set aside to cool.
Roughly break the biscuits. Fold into the dark chocolate mixture with the white chocolate and fruit and nut mix. Spoon into a lined 20cm (8 in) square tin, smooth the surface, then scatter over the marshmallows.
Cover and place in the fridge for 3 hours or until set. Using a sharp knife cut into 9 squares, then halve each square to make triangles. Keep cool.