Combine flour, 1 cup brown sugar and softened butter in large mixer bowl. Beat at medium speed of electric mixer 2 to 3 minutes or until mixture resembles fine crumbs. Pat mixture evenly onto bottom of ungreased 13 x 9 inch baking pan. Sprinkle pecans evenly over crumb mixture.
Combine 2/3 cup butter and remaining 1/2 cup brown sugar in small saucepan. cook and stir over medium heat until entire surface is bubbly. cook and stir 1/2 to 1 minute more. Pour into pan, spreading evenly over crust.
Bake in 350 degrees F (175 degrees C) oven 18 to 20 minutes or until entire surface is bubbly. Remove from oven; immediately sprinkle with chocolate pieces. Let stand 2 to 3 minutes to allow chocolate to melt; use knife to swirl chocolate slightly.
Cool completely in pan on wire rack. Use sharp knife to cut into 48 bars.