Step 1Heat the oven to 350 degrees. In a large bowl with an electric mixer, or in the bowl of a stand mixer, beat the graham cracker crumbs, butter, sugar and flour until moist and well-blended. Press the mixture firmly and evenly over the bottom of a 13-inch by 9-inch baking pan. Bake until the crust is golden brown, 10 to 15 minutes.
Step 1While the crust is baking, in a large bowl, whisk together the brown sugar and eggs to blend. Whisk in the graham cracker crumbs, vanilla, salt and baking powder until well-blended. Stir in the pecans.
Step 2Spread the mixture over the baked crust and return to the 350-degree oven until the filling is dark-golden on top and jiggles slightly when tapped, 20 to 25 minutes. Transfer the pan to a cooling rack and cool completely.
Step 3Sprinkle a light coating of sifted powdered sugar over the pan if desired, and cut into 24 bars. The bars can be made 1 day in advance. Wrap in plastic and keep at room temperature.