Prep the pan. Line a 9×13-inch metal baking pan with parchment, leaving overhang on the long sides. Lightly grease the parchment for easy release.
Brown the butter. In a saucepan over medium heat, cook 1 cup butter, stirring, until it foams and the milk solids turn deep golden and smell nutty, 5–7 minutes. Immediately pour into a heatproof bowl to stop cooking. Cool 10 minutes.
Make the crust. Whisk flour, salt, granulated sugar, and brown sugar. Stir in the slightly cooled brown butter and vanilla until a dough forms. It will look like damp sand that clumps when pressed—perfect.
Press and par-bake. Press dough evenly into the pan. Dock with a fork a few times. Bake at 350°F (175°C) for 15–18 minutes until set and lightly golden at the edges.
Mix the filling. In a large bowl, whisk eggs until homogenous. Add brown sugar, granulated sugar, corn syrup, melted butter, vanilla, bourbon, and salt. Whisk until smooth. Fold in chopped pecans.
Fill and bake. Pour filling over the hot crust. Smooth, then arrange pecan halves on top if using. Bake 22–28 minutes until the center has a slight jiggle but is not liquid. The edges should be set and glossy.
Cool completely. Transfer the pan to a rack and cool at least 2 hours. For clean slices, chill 1 hour before cutting. Sprinkle flaky salt if using.
Slice and serve. Use the parchment tabs to lift. Cut into 24 small bars or 16 larger squares. Flex responsibly.