Heat oven to 350°F. Line an 8-inch square pan with parchment, leaving overhang. Lightly grease.
Melt the butter and let it cool for 5 minutes so it’s warm, not hot.
Whisk brown sugar and granulated sugar into the butter until glossy and well combined.
Beat in the eggs and vanilla until the mixture looks smooth and slightly thick.
In a bowl, whisk flour, baking powder, and salt. Fold the dry mix into the wet just until no flour streaks remain.
Stir in white chocolate and nuts, if using. The batter will be thick.
Scrape batter into the pan and smooth the top evenly.
Bake 22–27 minutes, until edges are set and a tester comes out with a few moist crumbs.
Cool in the pan for 20 minutes. Lift out with parchment, cool fully, then cut into squares.