1. Heat oven to 350°F.
2. Combine flour and 1 cup brown sugar in large bowl; with pastry blender or fork, cut in 1/2 cup butter until fine crumbs form. Press mixture into 13x9-inch baking pan; sprinkle with pecans.
3. Combine remaining 2/3 cup butter and remaining 1/3 cup brown sugar in small saucepan; cook over medium heat, stirring constantly, until mixture boils. Continue boiling, stirring constantly, 30 seconds, until sugar dissolves; drizzle evenly over pecans and crust.
4. Bake 18 to 22 minutes until topping is bubbly and golden; remove from oven. Immediately sprinkle chocolates pieces over top. Cool completely in pan on wire rack; cut into bars. Makes 24 to 36 bars.