01 - Preheat oven to 350°F. Line 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 for topping.
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 well combined.
05 - Stir ground black sesame into wet mixture until fully incorporated.
06 - Add dry ingredients to wet mixture and fold gently until just combined.
07 - Fold in white chocolate chips.
08 - Pour batter into prepared pan and smooth top. Sprinkle with reserved whole black sesame seeds.
09 - Bake for 22-25 minutes until edges are golden and toothpick inserted into center comes out with a few moist crumbs.
10 - Let cool completely in pan before lifting out and cutting into squares.