Line a square baking dish with parchment paper.
Melt the dark chocolate and butter in a bowl over simmering water.
Stir until smooth and fully combined.
Remove from heat and let cool slightly.
In a large mixing bowl, combine mini marshmallows, mixed nuts, and crushed biscuits.
Pour the melted chocolate over the mixture and stir until well coated.
Transfer the mixture into the prepared baking dish.
Press down evenly to create a compact layer.
Chill in the refrigerator for at least 2 hours until set.
Once set, cut into squares and serve.