Mix together the flour, brown sugar and butter until it resembles fine crumbs. Press into the bottom of a 9x13-inch baking dish. Sprinkle the pecans over the top.
In a medium saucepan, combine the 2/3 cup butter and 1/2 cup brown sugar. Cook over medium heat until it comes to a full boil, stirring constantly. Continue to boil for 1 minute, then remove from heat and pour evenly over the crust.
Bake for 18-22 minutes, or until the top layer is bubbly. Remove from the oven and sprinkle with the chocolate chips. Allow to sit for about 5 minutes, until the chocolate has melted from the heat of the bars. Carefully spread the chocolate into a thin layer over the top of the bars. Cool completely before cutting into bars.