Preheat your oven to 175°C (350°F). Line a 9-inch square baking pan with parchment paper for easy removal.
Melt the butter gently in a microwave or on the stovetop until just melted and slightly cooled, then transfer to a mixing bowl.
Add the packed brown sugar and vanilla extract to the melted butter, then whisk together until smooth and fragrant, with a slight sheen.
Crack in the eggs one at a time, whisking well after each addition until the batter is glossy, smooth, and slightly thickened.
Sift in the all-purpose flour and baking powder, then gently fold into the wet ingredients until just combined, avoiding overmixing.
If using, fold in chopped nuts or chocolate chips to add texture and flavor to your blondies.
Pour the batter into the prepared pan, spreading it evenly with a spatula and smoothing the top for a uniform surface.
Bake for 22–25 minutes, or until the edges are golden and a toothpick inserted into the center comes out with moist crumbs.
Remove from the oven, sprinkle a pinch of sea salt on top, and let cool in the pan for about 10 minutes.
Transfer the blondies to a wire rack to cool completely, or serve slightly warm for a gooey, comforting treat.
Slice into squares, enjoy immediately, or store in an airtight container for up to 3 days for best flavor and texture.
