Combine evaporated milk, butter, sugar and marshmallows in 2-quart bowl.
Microwave, uncovered, 2 to 3 minutes on high, or until marshmallows puff and mixture begins to boil.
Microwave 2 1/2 to 3 1/2 minutes on medium, uncovered, or until mixture boils and sugar dissolves completely.
Stir in chocolate chips, nuts and vanilla.
Beat until smooth.
Spread in buttered (8 x 8-inch) baking dish.
Cut into 1-inch squares.