In a medium saucepan, combine sugar, evaporated milk, and chocolate. Cook over medium heat, stirring constantly, until chocolate is melted and mixture is smooth.
Remove from heat and stir in butter and mint extract.
Pour mixture into an 8x8 inch baking dish. Refrigerate for at least 2 hours, or until firm.
Notes
For a richer flavor, use dark chocolate instead of unsweetened chocolate. You can also add chopped nuts or chocolate chips to the fudge.