Lightly butter a 9x13" pan. Place butter and brown sugar an a heavy, medium saucepan and bring to a boil over medium heat. Stir in 2 cups of pecans. Pour into the prepared dish and bake 15 minutes at 375. Remove from oven and sprinkle with the chocolate chips; when soft enough to spread, spread evenly over toffee. Spread remaining 1/2 cup pecans on top. Let cool completely, then break into pieces. Store in air-tight container.