1. Line a 9×13-inch baking pan with parchment paper, leaving some overhang on the long sides to create a sling. Lightly grease the parchment and any exposed sides of the pan. Preheat the oven to 350°F (175°C).

2. In a large mixing bowl, whisk together the melted butter, granulated sugar, and 1/2 cup light brown sugar (for the brownie base) until thick, smooth, and glossy.

3. Add the 4 eggs for the brownie base one at a time, whisking well after each addition, then whisk in 2 teaspoons vanilla extract until the mixture looks shiny and slightly thickened.

4. In a separate bowl, whisk together the flour, cocoa powder, and 1/2 teaspoon fine sea salt (for the brownie base) until no cocoa lumps remain.

5. Gently fold the dry ingredients into the wet brownie mixture using a spatula, mixing just until no dry streaks remain to keep the brownies fudgy rather than cakey.

6. Spread the brownie batter evenly into the prepared pan, smoothing the top. Bake for 20–22 minutes, or until the edges are just set and the center still looks slightly underbaked. Leave the oven at 350°F (175°C).

7. While the brownie layer bakes, start the pecan topping. In a medium saucepan over medium heat, melt the remaining 1/2 cup unsalted butter. Stir in 1 cup light brown sugar, the corn syrup, and the heavy cream. Cook, stirring frequently, until the mixture comes to a gentle simmer and looks smooth and glossy, about 3–4 minutes.

8. Remove the saucepan from the heat and let the mixture cool for about 5 minutes so it is warm but not piping hot.

9. In a medium bowl, whisk together the 2 eggs for the topping, 2 teaspoons vanilla extract, and 1/2 teaspoon fine sea salt.

10. Slowly pour the warm sugar mixture into the egg mixture in a thin stream, whisking constantly to temper the eggs so they do not scramble. Continue whisking until everything is fully combined and silky.

11. Stir in the chopped pecans until all of the nuts are evenly coated in the buttery caramel mixture.

12. Take the pan with the par-baked brownies out of the oven. Carefully pour the pecan topping over the brownie layer, using a spatula to spread the pecans into an even layer all the way to the corners.

13. Return the pan to the 350°F (175°C) oven and bake for 22–25 minutes, or until the topping is bubbling around the edges and the center looks set but still slightly jiggly when you gently shake the pan.

14. Remove the pan from the oven and place it on a wire rack. Cool the brownies completely at room temperature, then transfer the pan to the refrigerator for at least 1–2 hours to firm up for clean slices.

15. Use the parchment sling to lift the chilled brownies out of the pan onto a cutting board. If desired, sprinkle lightly with flaky sea salt. Cut into 16 squares with a sharp knife, wiping the blade between cuts for the neatest pieces. Serve slightly chilled or at room temperature.