01 - Preheat the oven to 350°F. Line an 8x8-inch baking pan with parchment paper and lightly grease with butter or cooking spray.
02 - In a medium bowl, whisk together flour, baking powder, and salt until evenly combined. Set aside.
03 - In a large bowl, stir melted butter and brown sugar until smooth and well combined. Whisk in the egg and vanilla extract until fully incorporated.
04 - Gradually add the dry ingredients to the wet mixture, stirring with a spatula or wooden spoon until just combined. Do not overmix.
05 - Gently fold in the blueberries and white chocolate chips if using, taking care not to crush the berries.
06 - Spread the batter evenly into the prepared pan. Bake for 28-32 minutes, or until edges are golden brown and a toothpick inserted into the center comes out with a few moist crumbs.
07 - Cool completely in the pan before lifting out using the parchment paper. Cut into 12 bars and serve.