Melt butter in top of double boiler over hot water. Add chocolate morsels; stir until melted, then remove from heat. Let cool. Add marshmallows; stir lightly.
Spread half of the coconut in a greased 9x9x2 - inch pan. Spread chocolate mixture over coconut in pan. Top with remaining coconut. Press down with a spoon. Refrigerate until set. When well chilled, cut into small squares.