In a large saucepan, melt the semisweet chocolate chips and cubed butter over low heat, stirring constantly until the mixture is completely smooth and combined. Remove from heat and allow to cool slightly.

Stir the pastel miniature marshmallows and toasted chopped walnuts into the melted chocolate mixture evenly, ensuring all pieces are well coated.

Divide the mixture into three equal portions. Place each portion on a separate piece of waxed paper and shape it into a 10-inch long roll using the waxed paper to help form a tight cylinder.

Roll each chocolate roll thoroughly in the flaked coconut so the roll is completely coated on the outside.

Wrap each roll tightly with waxed paper. Refrigerate the wrapped rolls for 2 hours or until firm.

After chilling, unwrap the rolls and slice crosswise into 1/2-inch thick slices. Serve immediately or store chilled until ready to enjoy.