Butter an 8-inch square pan. You can also line with waxed paper and then butter. Lightly butter sides of sauce pan.
Combine sugar, milk, corn syrup and chocolate chips in sauce pan. Cook over medium heat stirring constantly until reaches rolling boil. Place candy thermometer in sauce pan. Do not touch bottom of pan. Keep cooking until reaches 238 degrees Fahrenheit. Stir occasionally.
Once reached desired temperature pour into heat-proof bowl. Add butter. Cool to 110 degrees Fahrenheit.
Once reached desired temperature add vanilla and optional walnuts. Beat with heavy duty mixer until thick and mixture loses gloss. Spread into prepared pan. Score. Refrigerate until firm. Cut into squares.
Chocolate Mint Fudge: Replace chocolate chips with mint flavor chocolate chips. Do not use vanilla.