Melt 1 cup (2 sticks) unsalted butter in a medium saucepan over medium heat. Swirl occasionally as it foams and crackles. After 5-6 minutes, milk solids turn golden amber with a nutty aroma. Immediately transfer to a heatproof bowl. Divide into two 1/2-cup portions.

Preheat oven to 350°F (175°C). Line a 9x13-inch metal baking pan with parchment paper, leaving an overhang on the sides.

In a medium bowl, whisk together flour, powdered sugar, and salt. Pour in the first 1/2-cup portion of brown butter. Mix until a crumbly dough forms. Press firmly and evenly into the prepared pan. Bake for 18-20 minutes until edges are lightly golden.

In a saucepan, combine remaining 1/2 cup brown butter, brown sugar, maple syrup, and heavy cream over medium heat. Bring to a gentle boil and simmer for 2-3 minutes until smooth. Remove from heat; stir in vanilla, salt, and chopped pecans.

Pour warm pecan filling over the warm par-baked crust. Smooth evenly and press whole pecan halves on top. Bake at 350°F for 22-25 minutes until bubbling and deep golden.

Sprinkle with flaky sea salt. Cool completely in pan, then chill in refrigerator for at least 1 hour. Lift out using parchment paper and slice into 24 squares with a sharp knife.