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 the all-purpose flour, baking powder, and salt until well combined. Set aside.
03 - In a large bowl, stir the melted butter and brown sugar until smooth and no sugar lumps remain. Whisk in the egg and vanilla extract until fully incorporated.
04 - Gradually add the dry ingredients to the wet ingredients, folding gently with a spatula just until combined. Do not overmix.
05 - Gently fold in the blueberries and white chocolate chips if using, taking care not to crush the berries.
06 - Transfer the batter to the prepared pan and spread evenly with an offset spatula or the back of a spoon.
07 - Bake for 28 to 32 minutes, or until the edges are golden brown and a toothpick inserted into the center comes out with a few moist crumbs attached.
08 - Allow the blondies to cool completely in the pan before lifting out by the parchment paper and cutting into 12 equal bars.