Grease a 15x10x1-in. pan with 2 teaspoons butter; set aside. In a large heavy saucepan, melt the remaining butter. Add sugar and water; cook and stir over medium heat until a candy thermometer reads 300° (hard-crack stage).
Quickly pour into prepared pan. Immediately sprinkle with chocolate chips. Allow chips to soften for a few minutes, then spread over toffee. Sprinkle with pecans. Let stand until set, about 1 hour. Break into bite-size pieces. Store in an airtight container.