In a medium mixing bowl, stir together flour and the 1 cup brown sugar. Using a pastry blender, cut in the 1/2 cup butter until the mixture resembles coarse crumbs. Press the crumb mixture into the bottom of an ungreased 13x9x2-inch baking pan. Bake crust in a 350 degree F oven for 10 minutes or until very lightly browned.
In a small saucepan, combine the 2/3 cup butter and the 1/2 cup brown sugar. Cook and stir until mixture boils. Boil gently for 1 minute. Remove filling from heat. Sprinkle pecans over partially baked crust. Pour filling over pecans.
Bake bars for 15 minutes more or until golden and mixture bubbles over entire surface. (Don'toverbake, or bars can become too hard after they cool.) Transfer baking pan to a wire rack.
Immediately sprinkle bars with chocolate pieces. Cool for 10 minutes. (If you like, you can spread the softened chocolate pieces over the surface of the bars.) Loosen edges and cut into bars while warm. Makes 48 bars.