Preheat the oven to 350°F. Grease a baking pan and line it with parchment paper, preferably a 9-inch square mold.
In a large bowl, mix melted butter with brown sugar, white chocolate chips, vanilla extract, and the egg. Incorporate well to achieve a homogeneous mixture.
Add the flour and baking powder to the mixture. Mix well until all the ingredients are fully incorporated.
Finally, add the fresh blueberries to the mixture and gently stir to distribute them evenly.
Pour the batter into the prepared mold and spread it evenly.
Bake in the preheated oven for 30-35 minutes or until the top is golden. Avoid over-baking to maintain the moistness of the brownies.
Once baked, remove from the oven and let it cool in the pan for 15 minutes.
Transfer the blondies to a rack to cool completely.
Optionally, add white chocolate chips and blueberry reduction or sauce on top.
Once the blondies are completely cool, cut them into squares.