Preheat oven to 350°. Place pecans in a single layer on a baking sheet, and bake 8 to 10 minutes or until toasted and fragrant, stirring after 5 minutes.
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.
Pulse 2 cups flour, toasted pecans, 1/2 cup powdered sugar, and 1 cup butter in a food processor 5 to 6 times or until mixture is crumbly. Press mixture onto bottom of prepared pan.
Bake at 350° on an oven rack one-third up from bottom of oven 25 minutes or just until golden brown.
Whisk together eggs and next 3 ingredients in a large bowl until blended. Combine baking powder, salt, and remaining 1/4 cup flour; whisk into egg mixture until blended. Pour lemon mixture into prepared crust.
Bake at 350° on middle oven rack 30 to 35 minutes or until filling is set. Let cool in pan on a wire rack 30 minutes. Lift from pan onto wire rack, using foil sides as handles, and let cool 30 minutes or until completely cool. Remove foil, and cut into 24 (2-inch) squares; sprinkle with powdered sugar. Garnish, if desired.