In a heavy saucepan, combine the sugar, water, and butter; mix well. Place on medium heat; don't stir after mixture starts to heat. Cook with lid on until mixture boils. Take lid off and cook to 248 degrees F.* on candy thermometer. Wash crystals down three or four times. At 248 degrees F.* add 2 drops green color. Cook to 258 degrees F.* Pour out candy to cool on a greased marble slab. Pour on mint flavoring. Let cool to lukewarm. Roll jelly-roll fashion. Pick up in hands and pull like taffy until desired consistency. Stretch into a rope about 1 inch in diameter onto a board covered with cornstarch. Cut immediately into 1-inch pieces (if candy gets hard, you can't cut it). Let stand on board to dry about an hour. Store in can with tight-fitting lid to ripen (about 12 to 24 hours). Variation: Use yellow color instead of green, rum extract instead of mint.