Preheat Oven & Prep Pan: Preheat your oven to 350°F (175°C). Line a 9x13-inch metal baking pan with parchment paper, leaving generous overhang on the sides for easy lifting later.
Mix & Par-Bake Shortbread: In a large mixing bowl, whisk together melted butter, granulated sugar, vanilla extract, and fine salt until emulsified. Fold in flour with a spatula until a soft, uniform dough forms. Press dough firmly into an even layer in the bottom of the prepared pan. Prick all over with a fork and bake for 18 to 20 minutes until lightly golden around the edges.
Simmer Gooey Caramel: While the crust bakes, combine 1/2 cup butter, dark brown sugar, pure maple syrup, and heavy cream in a medium saucepan over medium heat. Bring to a rolling boil and simmer gently for 3 full minutes, stirring constantly with a silicone spatula.
Coat Pecans & Spread: Remove caramel saucepan from heat. Immediately stir in vanilla extract, fine sea salt, and toasted pecans until completely coated in glossy caramel syrup. Pour hot caramel pecan mixture over the warm par-baked crust and spread evenly to the edges.
Oven Caramelization: Return pan to oven and bake for 18 to 20 minutes until the caramel is actively bubbling in vigorous pockets across the entire center of the pan.
Cool, Chill & Slice: Remove from oven and immediately scatter flaky sea salt over the bubbling surface. Cool completely in the pan on a wire rack for 2 hours, then refrigerate for 1 hour for razor-sharp slices. Lift out using parchment paper handles and slice into 16 clean squares.