Preheat oven to 325°F (160°C). Mix softened butter and 1/2 cup brown sugar, then add flour and chopped pecans. Press into a 9x13 inch pan and bake for 22-25 minutes. Cool for 30 minutes.
Reduce oven temperature to 225°F (110°C). Melt 3 Tbsp butter with 3 Tbsp brown sugar, add 1 cup pecans, coat and sauté for 3-5 minutes. Toast pecans in oven for 20 minutes. Cool.
In a saucepan, bring half and half, butter, and brown sugar to a boil, then simmer for 1 minute. Remove from heat, whisk in powdered sugar and vanilla. Pour over crust and top with toasted pecans. Chill for 30-45 minutes.