from Baking Illustrated

1 cup unbleached all-purpose flour
1/3 cup packed light brown sugar
1/4 cup toasted pecans, chopped coarse
1 teaspoon salt
1/4 teaspoon baking powder
6 tablespoons cold unsalted butter, cut into ½-inch pieces

For the Pecan Filling:
1/2 cup packed light brown sugar
1/3 cup light corn syrup
4 tablespoons unsalted butter (½ stick), melted
2 teaspoons vanilla extract
1/2 teaspoon salt
1 large egg, lightly beaten
2 cups toasted pecans, chopped coarse

Roast pecans: Adjust oven rack to middle position and heat oven to 350 degrees. Spread the pecans (2 1/4 cups) onto a baking sheet and place in preheated oven 6-8 minutes until fragrant. Shake the baking sheet halfway through baking so the nuts will toast evenly. Remove nuts from oven and set aside to cool completely.For the crust: Spray 9-inch square baking pan with nonstick cooking spray. Put a long piece of parchment paper in the bottom of the pan pushing it into corners and letting the parchment extend up the two short sides of the pan and overhang slightly on both ends. Fit second sheet of parchment paper in pan in same manner, perpendicular to first sheet. Spray sheets with nonstick cooking spray.
Place flour, brown sugar, pecans, salt, and baking powder in food processor. Process mixture until it resembles coarse cornmeal, about five 1-second pulses. Add butter and pulse until mixture resembles sand, about eight 1-second pulses. Pat mixture evenly into prepared pan and bake until crust is light brown and springs back when touched, about 20 minutes.For the pecan filling: While crust bakes, whisk together brown sugar, melted butter, corn syrup, vanilla, and salt in medium bowl until just combined. Add egg and whisk until incorporated.
Pour filling on top of hot crust and sprinkle pecans evenly over top. Bake until top is brown and cracks start to form across surface, 22 to 25 minutes. Cool on wire rack for 1 hour. Remove bars from pan using parchment handles and transfer to cutting board. Cut into bars that measure 1½ inches by 2¼ inches.