Preheat the oven to 350 degrees. Spray an 8 x 8 inch pan with cooking spray.
Melt the butter over medium heat on the stovetop. Once melted, pour it into a large mixing bowl and set aside to cool for about 15 minutes.
Add the brown sugar and mix with a wooden spoon. Stir in the egg and vanilla until completely combined.
Add the flour, baking powder, and salt and mix until just combined.
Fold in the white chocolate chips.
Using a spatula, spread the blondie batter into the pan. Arrange the berries cut-side down evenly over the top of the batter, gently pressing them down.
Bake at 350 degrees for 25-30 minutes. The edges should be firm, with the center being moist. Allow to cool on a wire rack, and then cut into squares for serving.