Preheat your 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 large bowl, whisk melted butter, granulated sugar, light brown sugar, vanilla extract, and fine sea salt until smooth. Stir in flour with a spatula until a thick dough forms. Press evenly into the bottom of the prepared pan. Bake for 18 to 20 minutes until the edges are lightly golden and set. Remove from oven and let sit while preparing the filling.

Melt 1/2 cup (1 stick) unsalted butter in a medium stainless steel saucepan over medium heat. Swirl constantly as the butter melts, foams, and crackles. After 4 to 5 minutes, golden-brown flecks will settle on the bottom and a nutty caramel aroma will develop. Immediately remove from heat to prevent scorching.

Whisk dark brown sugar, pure maple syrup, heavy cream, vanilla extract, and fine sea salt directly into the warm brown butter pan. Place over medium-low heat and simmer gently for 2 minutes, whisking until sugar dissolves into a glossy, bubbling caramel sauce. Remove from heat and fold in the chopped toasted pecans until evenly coated.

Pour the warm caramelized pecan filling evenly over the warm par-baked shortbread crust, smoothing it to the edges with an offset spatula. Return to the 350°F (175°C) oven and bake for 20 to 22 minutes until the filling bubbles vigorously across the entire surface.

Transfer the pan to a wire cooling rack and immediately sprinkle flaky Maldon sea salt across the top. Allow bars to cool completely at room temperature for 2 hours, then refrigerate for 1 hour to set the caramel filling firmly. Lift parchment out of the pan and slice into 16 clean, neat squares using a sharp chef's knife.