Melt the butter and chocolate chips in a large saucepan over low heat. Let the mixture cool slightly for 5-6 minutes. You want the mixture warm, but not warm enough to melt the marshmallows.
Stir the walnuts into the chocolate until coated completely. Stir the marshmallows into the chocolate mixture until the marshmallows are covered in chocolate. Let the mixture cool and start to harden up.
Spread one cup of the coconut on a large piece of parchment paper. Divide the cooled chocolate mixture into two even portions. Place one portion of the chocolate onto the coconut in a long log form. Sprinkle some of the coconut on top of the chocolate roll. Use the parchment paper over top of the chocolate to press it into a log shape, rolling in the coconut until there is enough coconut on the outside that it doesn't stick to your hands.
Wrap the coconut-covered chocolate roll in waxed paper, then plastic wrap to seal it. Refrigerate overnight or longer. Repeat with the remaining portion of the chocolate mixture.
To serve, defrost and cut into slices that are ⅓ of an inch thick. This is best kept refrigerated.
Notes
Nutritional values will vary due to brands of products used.