LINE baking sheets with wax paper.
COMBINE morsels and sweetened condensed milk in large, uncovered, microwave-safe bowl on HIGH (100%) power for 1 minute; STIR. The morsels may retain some of their original shape. If necessary, microwave at additional 10 to 15-second intervals, stirring just until morsels are melted. Stir in marshmallows, nuts and vanilla extract.
DROP by heaping tablespoon in mounds onto prepared baking sheets. Refrigerate until firm.