Preheat your oven to 350 F. Spread the chopped pistachios on a baking sheet and toast for 5-7 minutes until fragrant and lightly golden. Let them cool completely, then chop roughly.
Keep the oven at 350 F. Line an 8x8-inch pan with parchment paper, leaving an overhang for easy lifting.
In a large bowl, whisk together the melted butter and brown sugar until smooth and glossy. Add the eggs, yolk and vanilla, whisking until combined.
In a separate bowl, whisk together the flour, cornstarch, baking powder and salt.
Fold the dry ingredients into the wet mixture until just combined and no flour shows (don’t over mix!). Gently fold in the toasted pistachios and white chocolate.
Pour the batter into the prepared pan and smooth the top. Sprinkle extra pistachios and white chocolate on top for a pretty finish.
Bake for 27-30 minutes, or until the edges are golden and a toothpick inserted in the center comes out with just a few moist crumbs. Let cool completely in the pan before slicing into squares. Best left overnight at room temperature (they’ll firm up as they cool).