Combine flour and brown sugar in large bowl. With pastry blender or fork, cut in butter until fine crumbs form (a few large crumbs may remain). Press mixture onto bottom of ungreased 13x9x2-inch baking pan. Sprinkle pecans over crust.
Prepare TOFFEE TOPPING: Combine 2/3 cup butter and 1/3 cup packed light brown sugar in small saucepan; cook over medium heat, stirring constantly, until mixture comes to a boil. Continue boiling, stirring constantly, 30 seconds. Use immediately. Drizzle evenly over pecans and crust.
Bake 20 to 22 minutes or until topping is bubbly and golden; remove from oven. Immediately sprinkle milk chocolate chips evenly over top; press gently onto surface. Cool completely in pan on wire rack. Cut into bars. About 36 bars.