Preheat the oven to 350°F (175°C).

Line an 8x8-inch baking pan with parchment paper, leaving an overhang on the sides for easy removal.

In a large mixing bowl, whisk the melted butter, light brown sugar, and granulated sugar until smooth and fully combined, about 2 to 3 minutes.

Add the eggs, egg yolk, and vanilla extract. Whisk until fully incorporated and smooth.

In a separate bowl, whisk together the all-purpose flour, cornstarch, baking powder, and salt.

Add about three-fourths of the dry ingredient mixture to the wet ingredients. Mix with a spatula or wooden spoon until mostly combined. A few streaks of flour should remain.

Toss the raspberries and white chocolate chips or chopped white chocolate with the remaining dry ingredient mixture.

Gently fold the coated raspberries and white chocolate into the batter until no visible streaks of flour remain. Avoid overmixing to prevent breaking down the raspberries.

Pour the batter into the prepared baking pan and spread it evenly with a spatula.

Bake for 25 to 30 minutes, or until the top is shiny and crackly, the edges are lightly browned, and a toothpick inserted into the center comes out with a few moist crumbs.

Allow the blondies to cool completely in the pan before removing them.

For cleaner slices, refrigerate the blondies for 2 hours before cutting.

Lift the blondies from the pan using the parchment paper overhang. Slice into 16 squares. Drizzle with extra melted white chocolate, if desired, before serving.