Butter a large baking sheet; set aside.
Combine sugar and water in a 3-quart heavy saucepan. Bring to a boil, stirring gently to dissolve sugar. Wash down crystals on sides of pan with a small brush dipped in hot water. Insert a candy thermometer. Bring mixture to a boil over medium-high heat; cook until thermometer reaches 280 degrees, washing down crystals occasionally.
Reduce heat to medium; stir in butter and almonds. Mixture will crystallize, but continue to cook until mixture melts and thermometer registers 306 degrees, stirring often to blend melting sugar with crystallized sugar. Quickly pour mixture onto prepared baking sheet. Let cool. When hardened, break into pieces. Store at room temperature in an airtight container.