Preheat oven to 350°. Line bottom and sides of a 13- x 9-inch pan with heavy-duty aluminum foil, allowing 2 to 3 inches to extend over sides. Lightly grease foil.
Bake pecans in a single layer in a shallow pan 8 to 10 minutes or until lightly toasted and fragrant, stirring halfway through.
Pulse flour, powdered sugar, and 3/4 cup butter in a food processor 5 to 6 times or until mixture resembles coarse meal. Press mixture on bottom and 3/4 inch up sides of prepared pan.
Bake at 350° for 20 minutes or until edges are lightly browned. Cool completely on a wire rack (about 15 minutes).
Bring brown sugar and next 3 ingredients to a boil in a 3-qt. saucepan over medium-high heat. Stir in toasted pecans, and spoon hot filling into prepared crust.
Bake at 350° for 25 to 30 minutes or until golden and bubbly. Cool completely on a wire rack (about 30 minutes). Lift baked bars from pan, using foil sides as handles. Transfer to a cutting board; cut into bars.