In a large saucepan, melt chocolate chips and butter over low heat; stir until smooth. Cool slightly. Stir in marshmallows and walnuts.
Divide mixture into three portions; place each portion on a piece of waxed paper. Using waxed paper, shape each into a 10-in.-long roll; roll in coconut. Wrap tightly in waxed paper; refrigerate 2 hours or until firm. Cut crosswise into 1/2-in. slices.
To toast nuts, bake in a shallow pan in a 350° oven for 5-10 minutes or cook in a skillet over low heat until lightly browned, stirring occasionally.