In a light-colored saucepan over medium heat, melt the cubed butter. Swirl frequently as it foams, sizzles, and bubbles. After 5 to 7 minutes, amber-golden milk solids will form at the bottom with a deeply nutty aroma. Immediately pour into a heatproof mixing bowl, scraping all golden brown bits, and let cool for 10 minutes.

Preheat your oven to 350°F (175°C). Line an 8x8-inch or 9x9-inch square metal baking pan with parchment paper, leaving an overhang on the sides for easy lifting.

Add the dark brown sugar and granulated sugar to the warm brown butter. Whisk vigorously for 2 full minutes. Whisk in the eggs, egg yolk, and vanilla extract for another 2 to 3 minutes until the batter becomes pale, thick, and glossy (this vigorous whisking creates the signature paper-thin crackly top!).

Add the flour, salt, and baking powder. Switch to a silicone spatula and gently fold just until a few flour streaks remain. Fold in 1 1/4 cups of the toasted chopped pecans until evenly distributed (do not overmix!).

Spread the thick glossy batter into the prepared baking pan, smoothing the top. Scatter the remaining 1/4 cup toasted pecans and white chocolate chips over the surface.

Bake at 350°F (175°C) for 26 to 29 minutes, until the edges are golden and lightly puffed, the top is shiny and crackled, and a toothpick inserted in the center comes out with a few moist, fudgy crumbs (do not overbake to maintain maximum chewiness!).

Immediately sprinkle flaky Maldon sea salt over the hot crackly surface. Let the blondies cool completely in the pan on a wire rack for at least 1 hour before slicing into 16 fudgy squares.