Heat oven to 350°F. Line an 8-inch square pan with parchment and lightly grease.
Whisk melted butter, brown sugar, and granulated sugar in a large bowl until smooth and glossy.
Add eggs and vanilla. Whisk until fully blended and slightly thick.
Stir in flour, baking powder, and salt just until no dry streaks remain. Do not overmix.
Fold in shredded coconut and white chocolate chips, if using.
Spread batter evenly in the pan. Sprinkle extra coconut on top, if desired.
Bake 22–26 minutes, until golden at the edges and a tester has a few moist crumbs.
Cool in the pan, then lift out and cut into squares.