Preheat your oven to 350°F (175°C). Line a 9x13 inch baking pan with parchment paper, leaving some overhang for easy removal. Lightly grease the parchment with butter or cooking spray. Pro tip: The parchment overhang acts like handles – it makes removing the brownies so much easier!

In a medium saucepan over low heat, melt the butter and chopped dark chocolate together, stirring frequently until smooth and combined. Remove from heat and let cool for 5 minutes. Pro tip: Keep the heat low to prevent the chocolate from seizing. If you prefer, you can do this step in the microwave in 30-second intervals.

In a large mixing bowl, whisk the granulated sugar into the warm chocolate mixture until well combined. Add the eggs one at a time, whisking well after each addition, then stir in the vanilla extract. Pro tip: Make sure the chocolate mixture isn't too hot when adding eggs, or they might scramble.

In a separate bowl, whisk together the flour, cocoa powder, and salt. Gradually fold this dry mixture into the chocolate mixture until just combined – don't overmix. Pro tip: Overmixing develops gluten and can make your brownies tough instead of fudgy.

Fold in 3/4 cup of the chopped pistachios, reserving the remaining 1/4 cup for topping. Pour the batter into your prepared pan and spread evenly. Pro tip: Use a spatula to gently level the batter without pressing down too hard.

Sprinkle the remaining pistachios on top and bake for 25-30 minutes, until a toothpick inserted in the center comes out with a few moist crumbs. Pro tip: For fudgy brownies, don't overbake! The toothpick should not come out completely clean.

Let the brownies cool completely in the pan on a wire rack, about 2 hours. Use the parchment overhang to lift them out, then cut into squares with a sharp knife. Pro tip: For clean cuts, wipe your knife between each slice, or run it under warm water and dry it off.