01 - Preheat oven to 350°F. Line an 8x8-inch baking pan with parchment paper, leaving overhang for easy removal.
02 - Toast black sesame seeds in dry skillet over medium heat for 2–3 minutes until fragrant. Cool slightly, then grind 1/3 cup in spice grinder until fine. Reserve 2 tablespoons whole seeds.
03 - Whisk together flour, baking powder, and salt in medium bowl.
04 - Whisk melted butter and brown sugar in large bowl until smooth. Add egg and vanilla, whisking until combined.
05 - Stir ground black sesame into wet mixture until fully incorporated.
06 - Add dry ingredients to wet mixture. Fold gently until just combined.
07 - Fold in white chocolate chips.
08 - Pour batter into prepared pan, smooth top, and sprinkle with reserved whole sesame seeds.
09 - Bake for 22–25 minutes until edges are golden and toothpick inserted in center comes out with moist crumbs.
10 - Cool completely in pan before lifting out and cutting into squares.