Preheat your oven to 175°C (350°F) and grease a 9-inch square baking pan or line it with parchment paper.
In a saucepan, melt the butter over medium heat until it just begins to bubble and releases a nutty aroma, about 2 minutes. Pour the melted butter into a mixing bowl and let it cool slightly.
Add the brown sugar and white sugar to the cooled melted butter. Whisk vigorously until the mixture is smooth, shiny, and well combined, about 1 minute. The mixture should feel silky and look slightly glossy.
Stir in the vanilla extract. Then, crack in the egg and whisk until the batter is glossy, smooth, and slightly thickened, about 1 minute. This helps the mixture relax and emulsify.
In a separate bowl, sift together the flour to avoid lumps, then gradually fold it into the wet mixture using a spatula or whisk. Mix just until combined; do not overmix to keep the blondies tender.
If using, fold in the chocolate chips or chopped nuts. Spread the batter evenly in the prepared pan with a spatula, smoothing the surface gently.
Bake in the middle of the oven for 22–25 minutes. The edges should turn a light golden brown, and a toothpick inserted into the center should come out with moist crumbs, not wet batter.
Remove the blondies from the oven and let them cool in the pan for about 15 minutes. This allows them to set and makes cutting easier.
Transfer the blondies to a wire rack or cut directly in the pan. Slice into squares and serve warm or at room temperature for a soft, chewy treat with a buttery vanilla aroma.
