Par-Bake the Shortbread Base: Preheat oven to 350°F (175°C). Line a 9x13-inch metal baking pan with parchment paper, leaving generous overhang on the sides. In a mixing bowl, combine 1 cup melted butter, 1/2 cup powdered sugar, 2 cups flour, and 1/2 teaspoon salt. Press the dough evenly across the bottom of the prepared pan. Prick all over with a fork. Bake for 18 to 20 minutes until edges are lightly golden.
Brown the Butter: While the crust bakes, melt 1/2 cup unsalted butter in a medium saucepan over medium heat. Swirl occasionally as it foams and crackles. After 4 to 5 minutes, amber flecks will appear and the butter will release a nutty hazelnut aroma. Immediately remove from heat and transfer to a heatproof bowl.
Whisk Pecan Caramel Filling: Whisk dark brown sugar, maple syrup, heavy cream, vanilla extract, cinnamon, and 1/2 teaspoon salt into the warm brown butter until smooth and emulsified. Whisk in the beaten eggs one at a time. Fold in 3 cups of pecans until evenly coated in caramel.
Bake to Golden Perfection: Pour the warm pecan mixture directly over the warm par-baked shortbread crust. Spread evenly with an offset spatula. Bake at 350°F for 24 to 28 minutes until bubbly across the surface with a slight set wobble in the center.
Cool, Garnish & Slice: Remove from oven and immediately scatter flaky sea salt across the top. Cool completely in the pan for 2 hours, then refrigerate for 1 hour. Lift out using the parchment wings and slice into 24 neat bakery bars.