Melt butter & chocolate in a bowl over pot with hot water or double boiler. Remove from heat and whisk in egg until combined. Cool for a few minutes then gently mix in icing sugar and marshmallows.

Meanwhile sprinkle a large piece of tin foil with coconut. Pour the chocolate mixture over the coconut in a long log like shape. You will need to get your hands dirty to help mold it into an evenly shaped log. Roll like a jelly roll/log in the coconut. Wrap & seal entire log in aluminum foil and keep in freezer until ready to serve.