Melt butter and chocolate chips in a pan over low heat. Let mixture cool slightly before stirring in walnuts and marshmallows. Set aside to cool completely.

Spread coconut on waxed paper.

Divide cooled dough into two even portions. Form each portion into a long roll. Coat rolls with coconut.

Wrap rolls in waxed paper and then again in plastic wrap. Refrigerate overnight or longer.

Cut into 1/2-inch-thick slices to serve.