Preheat the oven to 350°Fahrenheit/175°Celsius. Grease and line an 8x8 baking pan, set aside.
In a mixing bowl, whisk together the cooled melted butter and light brown sugar. Once smooth, add in your egg, egg yolk, and vanilla extract. Whisk once more.
Pour in the flour, baking powder, salt and cornstarch. Use a rubber spatula to fold everything together until there are no more streaks of flour.
Add in your frozen raspberries and chocolate chips (though keep a small handful of both raspberries and chocolate chips to the side), folding gently so the raspberries don't break apart.
Pour the blondie batter into your prepared pan. Use the rubber spatula to even out the top. Sprinkle over the reserved chocolate chips and raspberries. Bake for 32-36 minutes, they should be a golden brown, and set. Allow them to cool completely before slicing into 9 or 12 bars.
Notes
My recipes have been developed and tested in metric grams. I highly recommend using a weighing scale and measuring out your ingredients the metric way. Room Temperature: I make sure that all of my ingredients are at room temperature before I start the recipe, so pull out all of the ingredients before you start baking.Butter: I use European style butter, like Kerry Gold, in my recipes. Melt the butter and let it cool completely before baking. Salt: If you don’t have kosher salt, halve the amount of salt called for in the recipe. 1 teaspoon kosher salt = ½ teaspoon fine salt.Raspberries: You can use fresh or frozen raspberries. If you're using frozen, don't let them defrost before baking. Storage: Keep tightly wrapped in the at room temperature for 3 days or in the fridge for up to 1 week. You can tightly wrap the blondies in plastic wrap and freeze for up to 3 months.