1 cup butter
1 1/2 cups granulated sugar
1/4 teaspoon cream of tartar
6 ounces bittersweet (not unsweetened) or
    semisweet chocolate, finely chopped
1 cup coarsely chopped pecans

Line a 9-inch square baking pan with foil, overlapping sides. Butter foil. Melt the 1 cup butter in a heavy medium saucepan over medium heat. Add sugar and cream of tartar and stir until sugar dissolves. Increase heat to medium-high. Brush down sides of pan with a wet pastry brush. Cook until mixture registers 310 degrees F on a candy thermometer, stirring occasionally, about 11 minutes.

Immediately pour toffee into prepared pan. Let stand 1 minute. Sprinkle with chocolate. Let stand 2 minutes to soften. Spread chocolate with the back of a spoon over toffee until melted and smooth. Sprinkle with pecans. Refrigerate until firm.

Remove toffee from pan, using foil as an aid. Break into 3-inch pieces. This can be prepared four days ahead, then chilled in an airtight container.