Brown the butter. Preheat oven to 170°C (340°F). In a small saucepan over medium heat, melt your butter. Continue to cook until the butter is golden brown and has a nutty fragrance. This takes about 4-5 minutes. Note that the butter turns very quickly from golden to burnt, so keep an eye on it. Remove from the heat, pour into a large mixing bowl, and leave at room temperature cool for ten minutes.
Prepare your pan. Line a 23cm (9-inch) square baking pan with parchment paper. Leave an overhang on the sides so that it's easy to lift the blondies out after they are baked.
Mix sugar into the butter. Into the bowl with the brown butter, add the light brown sugar, and whisk until well mixed in.
Add eggs. Add the 2 eggs, the 1 egg yolk, and the vanilla extract into the mixing bowl, and using a hand-held electric whisk on high speed, whip until the mixture is visibly paler and thicker. This can take 3 to 4 minutes to achieve. The length of time not only cools the butter down but also aerates the eggs creating that shiny crackly top.
Add dry ingredients. Sift into this mixture the flour, baking powder, and salt and fold through using a rubber spatula until a few flour streaks remain.
Add chocolate and raspberries. Fold in ¾ of the white chocolate chips and raspberries until just combined. Spoon batter into the prepared baking tray and level with an offset spatula. Sprinkle over the of top the remaining chocolate chips and raspberries. There's no need to press them in.
Bake. Bake for 25-30 minutes or until lightly browned on top and the sides are set, and a toothpick inserted into the centre comes out with a few moist crumbs. Rotate the pan halfway through baking. Remove from the oven and let sit in the pan for ten minutes. Using the paper sling, lift the blondie square out onto a wire rack and leave it to cool completely. Use a sharp knife, to cut into 16 squares and serve immediately.