Spray a 9x13" pan with non-stick spray and set aside. In mixing bowl, cream together 1 c. butter (softened, NOT melted), and 1/2 c. brown sugar. Add 1 1/2 c. flour, and 1/2 c. chopped pecans. Press mixture into pan, and bake at 325 for 22-25 minutes. Set aside to cool for 30 minutes.
Adjust oven temperature to 225 degrees. While crust cools, in small skillet, melt 3 Tbsp. butter with 3 Tbsp. of brown sugar. When melted add 1 1/2 c. pecan halves. Coat pecans with sugar mixture and saute for about 3-5 minutes on medium-low heat. Transfer the coated pecans to a baking sheet at toast in a 225 degree oven for 20 minutes. When done, set aside to cool.
In a saucepan, add 1/2 c. cream, 1/3 c. butter and 1 1/2 c. brown sugar. Bring to a low boil over medium heat while stirring. Boil for 1 minute and remove from heat. Whisk in powdered sugar and vanilla until smooth. Pour over pecan crust, and top with toasted pecans. Let stand 30-45 minutes to set-up. Store in refrigerator.