Line your 8x8 baking dish with parchment paper, letting the edges hang over for easy removal later. Preheat your oven to 350°F.
In a medium bowl, combine the melted butter, white sugar, and brown sugar. Whisk them together vigorously until the mixture is smooth and fully blended.
Crack in the egg and pour in the vanilla extract. Whisk again until everything is combined.
In a separate bowl, whisk together the flour, baking powder, and salt. This ensures the leavening agents are evenly distributed.
Gradually pour the dry ingredients into the wet mixture. Toss in the frozen raspberries at this point, too. Using a spatula, gently fold the mixture until it’s mostly combined.
Fold in the white chocolate chips until no streaks of flour remain and the chips are evenly dispersed. The batter will be thick—that’s exactly what you want.
Transfer the batter into your prepared baking dish, spreading it evenly. For an extra burst of color and flavor, scatter a few more raspberries and white chocolate chips on top.
Place the dish in your preheated oven and bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean.
Let the blondies cool in the pan for at least 15 minutes. This helps them set, making slicing much easier.
If you’re feeling fancy, drizzle melted white chocolate over the top for a final touch. Slice into squares, serve, and watch them disappear.