Line a square cake tin with baking paper, leaving some paper hanging over the edge of the tin to make the rocky road easier to lift out. Put the low-fat spread and chocolate in a large microwave-safe bowl and heat in the microwave for 1 minute, or until melted. Set aside for 5 minutes to cool slightly.

Reserve a few of the marshmallows for decoration, then add the rest to the chocolate mixture, along with the biscuits, and stir well to combine. Spoon the mixture in the prepared tin and smooth the top with a spatula. Scatter over the reserved marshmallows and chill for 1 hour or until set.

Lift the rocky road out of the tin, remove the baking paper and cut into 20 equal pieces.