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 distributed.
03 - In a large bowl, stir melted butter and brown sugar until smooth and no lumps remain. Whisk in the egg and vanilla extract until fully incorporated.
04 - Gradually add the dry ingredients to the wet ingredients, mixing with a spatula 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 in the prepared pan, using an offset spatula or the back of a spoon to smooth the top.
07 - Bake for 28-32 minutes, or until the edges are golden brown and a toothpick inserted into the center comes out with a few moist crumbs.
08 - Cool completely in the pan before lifting out using the parchment paper overhang and cutting into 12 bars.