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 inch baking pan. Sprinkle pecans evenly over crumb mixture. Combine two third cup butter and remaining half brown sugar in small saucepan. Cook and stir over medium heat until entire surface is bubbly. Cook and stir half 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.