Prep Oven & Pan: Preheat oven to 350°F (175°C). Line a 9x13-inch metal baking pan with parchment paper, leaving an overhang on the sides for easy lifting later.

Bake the Shortbread Crust: In a medium bowl, whisk together melted butter, granulated sugar, brown sugar, vanilla, and salt until smooth. Stir in flour with a rubber spatula until a soft dough forms. Press dough evenly into the bottom of the prepared pan. Bake for 18 to 20 minutes until edges are light golden brown.

Brown the Butter: While crust bakes, melt 1/2 cup butter in a saucepan over medium heat. Swirl pan continuously for 4 to 5 minutes as it foams, crackles, and develops amber specks on the bottom with a nutty fragrance. Immediately take off heat.

Simmer the Pecan Caramel: To the brown butter, whisk in dark brown sugar, pure maple syrup, heavy cream, vanilla, and kosher salt. Return to medium heat and simmer gently for 2 to 3 minutes until sugars dissolve and the mixture bubbles into a glossy caramel. Remove from heat and stir in chopped pecans.

Pour & Bake: Pour warm pecan filling over the warm par-baked shortbread crust, spreading pecans evenly to all four corners. Return to oven and bake at 350°F for 20 to 22 minutes until filling is bubbling across the center.

Cool, Slice & Garnish: Transfer pan to a wire rack and sprinkle flaky sea salt over the top. Allow to cool completely to room temperature (about 2 hours, or chill 1 hour in fridge). Lift out by parchment overhang, slice into 16 bars with a sharp chef knife, and serve!