Microwave chocolate morsels and sweetened condensed milk in a large, uncovered, microwave safe bowl on High (100%) power for 1 minute; Stir. Morsels may retain some of their original shape. If necessary, microwave an additional 10 to 15 second intervals, stirring just until morsels are melted.
Stir in vanilla extract.
Fold in marshmallows and nuts.
Press mixture into prepared baking pan. Refrigerate until firm.