Preheat the oven to 150c/300f, place the coconut on a baking tray and toast in the oven for 2-4 minutes – checking regularly until browned (be careful, it burns quickly). Take out of the oven and leave to cool
Place popcorn in a bowl.
Place the marshmallows, sugar and butter in a pan and heat on a low-to-medium heat until melted (stirring frequently). Pour the mixture over the popcorn and stir to combine (it will be very sticky). Sprinkle over the toasted coconut.