Preheat oven to 350 degrees F.
Lightly spray a 13 x 9 x 2 inch baking pan with vegetable cooking spray.
Beat 1/4 cup butter and 1/2 cup brown sugar at medium speed with an electric mixer until blended. Add egg yolk beating until blended. Add flour and salt; beat on low speed until blended. Press mixture into bottom of prepared pan. Bake crust 12 to 14 minutes or until golden. Cool in pan on a wire rack.
Combine 1/4 cup butter, brown sugar, corn syrup, evaporated milk and vanilla in medium saucepan. Cook over low heat, stirring constantly, until brown sugar dissolves.
Bring mixture to a boil over medium heat and cook until candy thermometer reaches or 265 degrees (about 10 minutes). Pour mixture over crust and spread evenly. Bake 10 additional minutes. Cool 5 minutes on a wire rack.
Combine chocolate squares and brown sugar in top of a double boiler; bring water to a boil. Cook until chocolate melts and mixture is blended. Drizzle (or spread) over toffee. Sprinkle with chopped pecans.
Cool completely on wire rack. When chocolate is firm, cut into bars or diamond shapes. Store in an airtight container.
Per serving: Total Calories: 161, Total Fat: 8 g, Cholesterol: 17 mg, Sodium: 32 mg, Total Carbohydrates: 19.8 g, Dietary Fiber: 0.9 g, Protein: 1.5 g