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 flour, baking powder, and salt until well blended.
03 - In a large bowl, stir the melted butter and brown sugar until smooth and creamy. Whisk in the egg and vanilla extract until fully incorporated.
04 - Gradually add the dry ingredients to the wet mixture, folding gently with a spatula just until combined. Avoid overmixing to keep the blondies tender.
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, ensuring the surface is level.
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.
08 - Let the blondies cool completely in the pan before lifting out using the parchment paper. Cut into 12 equal bars and serve.