Prep the pan and oven: Heat the oven to 350°F (175°C). Line a 9x13-inch metal baking pan with parchment, leaving overhang for easy removal.
Lightly grease the parchment.
Mix the crust: In a bowl, whisk melted butter, sugar, vanilla, and salt. Stir in the flour until a soft, slightly crumbly dough forms. It should hold together when pressed.
Press and pre-bake: Press the dough evenly into the lined pan, building a slight 1/2-inch rim up the sides to contain the filling.
Prick all over with a fork. Bake 15–18 minutes until just set and lightly golden at the edges.
Toast the pecans: While the crust bakes, spread pecans on a sheet pan and toast in the oven for 6–8 minutes, until fragrant. Cool slightly, then roughly chop if not already chopped.
Make the filling: In a large bowl, whisk eggs until smooth.
Add maple syrup, brown sugar, melted butter, vanilla, and salt. Whisk in flour until no dry streaks remain. Stir in the pecans.
Add bourbon or cinnamon if using.
Assemble: Pour the filling over the hot crust and spread evenly. If you saved pecan halves, arrange them on top in a simple pattern.
Bake: Return to the oven and bake 25–32 minutes, until the filling is set at the edges and jiggles slightly in the center. The top should look glossy and lightly browned.
Cool completely: Set the pan on a rack and let the bars cool fully, at least 2–3 hours.
For the cleanest slices, chill 1 hour after cooling.
Finish and slice: Lift out using the parchment. Sprinkle lightly with flaky sea salt if desired. Use a sharp knife to cut into 24 squares or 18 larger bars, wiping the blade between cuts.