Preheat oven to 350°F (175°C) and line a 9x13-inch baking pan with parchment paper.
Beat softened butter, brown sugar, flour, and salt until crumbly. Press firmly into the pan and bake for 18 minutes until lightly golden.
Whisk together the eggs, dark brown sugar, maple syrup, melted butter, vanilla, and cinnamon until smooth, then fold in the chopped pecans.
Pour the pecan filling over the warm pre-baked crust and bake for 25-28 minutes until set.
Sprinkle with flaky sea salt, cool completely for 2 hours, and slice into 20 bars.