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. In a medium bowl, stir together melted butter, granulated sugar, vanilla extract, and salt. Add the flour and stir until a soft, crumbly dough forms. Press evenly into the bottom of the prepared pan. Bake for 15 minutes until lightly golden around the edges.

While the crust bakes, melt 1/2 cup butter in a small light-colored saucepan over medium heat. Swirl the pan occasionally as the butter melts, foams, and crackles. After about 4 to 5 minutes, nutty brown flecks will form at the bottom and release an intoxicating hazelnut aroma. Immediately pour into a mixing bowl to prevent burning and cool for 5 minutes.

To the browned butter, whisk in the dark brown sugar, maple syrup, heavy cream, room-temperature eggs, vanilla extract, cinnamon, and kosher salt until completely smooth, glossy, and emulsified. Stir in the toasted chopped pecans until fully coated in caramel.

Pour the pecan caramel mixture directly over the warm par-baked shortbread crust. Spread into an even layer with an offset spatula. Bake at 350°F (175°C) for 22 to 25 minutes, until the filling is bubbling and the center is set with only a slight jiggle. Remove from oven and immediately sprinkle with flaky sea salt.

Allow the bars to cool completely to room temperature in the pan for 1 hour, then chill in the refrigerator for 1 hour to firmly set the caramel. Use the parchment handles to lift the chilled slab onto a cutting board. Slice into 16 to 24 neat squares using a sharp chef's knife wiped clean between cuts.