Line an 8in / 20cm square pan with aluminum foil. Crumble 75g of digestive biscuits and 25g of walnut pieces into a bowl. Snip 150g of pink and white marshmallows into halves or quarters and add them.
Chop 300g of dark chocolate and 50g of milk chocolate into a heatproof bowl. Gently melt 100g of butter with 2 tablespoons of golden syrup, pour it over the chocolate, and stir until smooth.
Pour the chocolate into the marshmallow mixture and stir until coated. Spread loosely in the pan without pressing down, leaving the top bumpy. Refrigerate for 1-2 hours, until set.
Dust with 1 teaspoon of icing sugar and cut into bars or squares.
