Grease a 15-in. x 10-in. x 1-in. pan with 2 teaspoons butter; set aside.
In a large heavy saucepan, combine sugar and cream of tartar; stir in cream and remaining butter. Cook and stir over medium heat until a candy thermometer reads 300° (hard-crack stage). Remove from the heat; stir in extract. Quickly pour into prepared pan. Let stand at room temperature until cool.
In a microwave, melt chocolate chips; stir until smooth. Spread over toffee. Sprinkle with nuts. Let stand until set, about 1 hour. Break into pieces. Store in an airtight container. Yield: 1-3/4 pounds.