1Grease 8-inch square baking dish with butter.
2In 2-quart microwavable bowl, stir condensed milk and chocolate chips; add baking chocolate. Microwave uncovered on High 1 minute; stir. Microwave about 2 minutes longer or until chocolate is melted and mixture can be stirred smooth.
3Stir in vanilla and nuts. Spread mixture evenly in baking dish. Refrigerate until firm. For 64 (1-inch) squares, cut into 8 rows by 8 rows.