Heat the oven to 350°F (175°C). Line an 8-inch square baking pan with parchment paper, leaving enough overhang to lift out the cooled blondies.

If desired, toast the pecans at 350°F (175°C) for 5 to 8 minutes. Cool them before chopping and adding them to the batter.

Melt the butter in a saucepan over medium heat. For browned butter, continue cooking until the milk solids turn golden and the butter smells nutty, watching closely so it does not burn; cool for about 10 minutes.

Whisk the brown sugar into the melted butter. Add the eggs and vanilla, then whisk until the mixture is smooth.

In a separate bowl, whisk together the flour, baking powder, and fine salt. Fold the dry mixture into the butter mixture just until no dry streaks remain.

Fold in the chopped pecans without overmixing. Spread the batter evenly in the prepared pan and sprinkle with flaky sea salt if desired.

Bake for 25 to 30 minutes, until the edges are set and lightly browned while the center still looks slightly soft. A toothpick should come out with moist crumbs rather than wet batter.

Cool the blondies completely in the pan for at least 30 to 60 minutes before lifting them out and cutting them into squares.