Line a lightly greased 8" square pan with nonstick aluminum foil; set aside.
Combine first 4 ingredients in a medium-size heavy saucepan. Cook over medium heat, stirring constantly, until mixture comes to a boil; boil 4 minutes, stirring constantly. Remove from heat. Add marshmallows, wasabi paste, if desired, plus chocolate morsels, and vanilla. Beat by hand with a wooden spoon until melted and smooth. Spread fudge into prepared pan. Sprinkle top with crystallized ginger, and press lightly into fudge. Refrigerate 2 hours or until firm. Cut into squares.