Adjust oven rack to lowest position and preheat to 350 degrees F. Spray a 9x13-inch pan with non-stick cooking spray and line with parchment paper.
Make the Crust: In a medium bowl, whisk together the flour, sugar, and salt. Add the melted butter and stir with a wooden spoon until a dough begins to form. Using your hands, continue to combine until no dry flour remains and a small portion of dough holds together when squeezed in the palm of your hand. Evenly scatter tablespoon-size pieces of dough over the surface of the pan. Using your fingertips, press and smooth dough into even layer in the bottom of the pan; set aside.
Make the Topping: In a medium bowl, whisk together the brown sugar, corn syrup, melted butter, vanilla, and salt until smooth and completely incorporated. Fold the pecans into the sugar mixture until the nuts are evenly coated.
Pour the pecan topping over the crust and, using a rubber spatula, spread the topping over the crust, working it to the edges and into corners (there will be bare patches). Bake until the topping is evenly distributed and rapidly bubbling across entire surface, 23 to 25 minutes.
Transfer the pan to a wire rack and lightly sprinkle with flake sea salt, if using. Let bars cool completely in the pan, about 1½ hours. Cut into 24 bars and serve. The bars can be stored at room temperature in an airtight container for up to 5 days.