Line an 8x8-inch (20x20cm) baking pan with parchment paper.

Melt chocolate and butter together in a microwave (30-sec bursts) or double boiler until smooth.

Stir in marshmallows, nuts, and crushed biscuits until fully coated.

Pour mixture into the pan, pressing down evenly.

Chill in the fridge for 2 hours or until firm.