Preheat oven to 450° F.
Cream butter in large bowl on medium speed for 2 minutes. Add light brown sugar; beat on medium for 2 minutes. Scrape sides of bowl with silicone spatula.
Add dark brown sugar; beat on medium for 2 minutes. Scrape sides of bowl with silicone spatula.
Add light Karo syrup; beat on medium for 2 minutes. Scrape sides and bottom of bowl with silicone spatula.
Add dark Karo syrup; beat on medium for 2 minutes. Scrape sides and bottom of bowl with silicone spatula.
Add vanilla extract; beat on medium for 2 minutes. Scrape sides and bottom of bowl with silicone spatula.
Add eggs, one at a time; beating until yolk disappears. Scrape sides and bottom of bowl with silicone spatula.
Add 1-½ cups of pecan pieces to pie shell. Ladle batter over pecans until mixture reaches the line in the shell where the fluting starts. Repeat with remaining shell.
Bake at 450° F for 10 minutes. Reduce heat to 350° F and continue baking for another 35 minutes.
Remove from oven; cool completely on wire rack.
*If using regular size pie shells, fill 16 mini pie shells with 1 tablespoon of chopped pecans and 2 tablespoons of batter. Bake at 325° F for 40 minutes. If using deep dish pie shells, fill shells with 2 cups of chopped pecans and divide batter between the two shells. Bake at 450° F for 10 minutes; reduce the heat to 350° F and continue baking for another 45 minutes. Cool completely.
Note: Can be prepared in advance, and frozen. Best served cold.