Combine flour, 1 cup brown sugar and softened butter in large mixer bowl. Beat at medium speed of electric mixer 2-3 minutes or until mixture resembles fine crumbs. Pat mixture evenly onto bottom of ungreased 13x9" baking pan. Sprinkle pecans evenly over crumb mixture. Combine 2/3 cup butter and remaining 1/2 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'F. oven 18-20 minutes or until entire surface is bubbly. Remove from oven; immediately sprinkle with chocolate pieces. Let stand 2-3 minutes to sllow chocolate to melt; use knife to swirl chocolate slightly. Cool completely in pan on wire rack. Use sharp knife to cut into 48 bars. Makes 48 bars.