Preheat your oven to 350°F (175°C) and line an 8x8-inch baking pan with parchment paper for easy removal.

In a medium saucepan over medium heat, melt the butter until it transforms into a deep amber color and releases a nutty aroma—about 5-7 minutes.

In a large mixing bowl, whisk the brown butter with light brown sugar and granulated sugar until smooth and glossy.

Incorporate the egg, egg yolk, and vanilla extract into the mixture, whisking until thick and glossy.

In a separate bowl, sift together the flour, baking powder, and sea salt. Gently fold into the wet mixture until no streaks of flour remain.

Pour the blondie batter into the prepared pan and spread it evenly. Bake for 22-25 minutes until the edges set and the center jiggles slightly.

Immediately sprinkle flaky sea salt on top after removing from the oven. Allow the blondies to cool in the pan for at least 20 minutes.