Preheat the oven to 350°F.
Line the bottom of an 8-inch or 9-inch baking pan with parchment paper, spray lightly with non-stick cooking spray.
Melt butter and let it cool down to slightly warm or room temperature.
Whisk together the cooled melted butter and brown sugar in a large mixing bowl, then whisk in the milk and vanilla.
In another bowl, add the flour, cornstarch, baking powder and salt and whisk together. Add to the wet mixture and mix everything together just until combined.
Fold in the white chocolate and pistachios.
Transfer the batter to the prepared baking pan, spreading the batter evenly on top.
Bake the blondies until set on top, for 8-inch pan bake 30-40 minutes and for the 9-inch pan, bake for 22 to 27 minutes.
Transfer the baking pan to a cooling rack and allow the blondies to cool before slicing and serving.