Combine brown sugar, evaporated milk and butter in an 8 cup Pyrex measuring cup; mix well. Microwave uncovered on High for 10 minutes, stirring at halftime. Stir in marshmallows and chips until melted. Add vanilla (and nuts) and beat until smooth. Spread evenly in a buttered 8" square pan. Refrigerate. Cut in squares. Yield: Approx. 25 pieces.