Preheat your oven to 350°F (175°C) and grease a square baking pan.

Melt the butter in a saucepan over low heat. Stir in granulated and brown sugars until combined.

Remove from heat, let cool slightly, then whisk in eggs one at a time, followed by vanilla extract.

In a separate bowl, mix flour and cocoa powder. Gradually fold this mixture into the wet ingredients until just combined.

Gently fold in chopped pistachios.

Pour the batter into the prepared pan and bake for 25-30 minutes or until a toothpick comes out with moist crumbs.

Cool before cutting into squares.