Preheat the oven to 350°F (177°C). Line a 9x9-inch baking pan with parchment paper, leaving an overhang on two sides for easy removal.
In a large mixing bowl, whisk together the melted butter, granulated sugar, and brown sugar until smooth and well combined.
Add the eggs, vanilla extract, lemon zest, and salt. Whisk until the mixture is smooth, glossy, and slightly lighter in color.
In a separate bowl, whisk together the flour and baking powder.
Add the dry ingredients to the wet ingredients and stir just until combined. Avoid overmixing.
Gently fold in the white chocolate chips. Then carefully fold in the frozen raspberries, being careful not to crush them too much.
Spread the batter evenly into the prepared baking pan. Sprinkle a few extra white chocolate chips and raspberries over the top, if desired.
Bake for 28–33 minutes, or until the edges are lightly golden and set. The center may still look slightly underdone, but will continue to set as the blondies cool.
Let the blondies cool in the pan for 15–20 minutes. Using the parchment paper overhang, lift them out of the pan and transfer to a wire rack to cool completely.
Slice into 16 squares and serve.