Reserve 50 g (2 oz) of the white marshmallows. Put 25 g (1 oz) of the butter and the remaining marshmallows into a saucepan and heat very gently until melted. Remove from the heat and stir in the cereal until evenly coated.
Spoon the mixture into an 18 cm (7 inch) square shallow baking tin, greased and lined with baking parchment (see
Place the remaining butter and reserved marshmallows in a small saucepan and heat gently until melted. Drizzle into the tin in lines, then scatter the sprinkles over the top. Leave in a cool place for 2 hours or until firm.
Turn out of the tin on to a board, peel off the lining paper and cut into small squares.