Add 1 cup of chocolate chips and softened butter to a heat-safe bowl and melt them together. Set aside.
In a large bowl, whisk sugar and eggs together until creamy.
Add flour, cocoa powder, sour cream, vanilla extract, ¼ cup of chocolate chips, and your butter/chocolate mixture and whisk until incorporated. You should have a smooth, lump-free batter. Set aside.
Grease your pan with butter or a non-stick spray, then add a dusting of flour. Pour your batter into the pan, spreading it evenly across the bottom. Sprinkle half of the pistachios across the top. Bake at 350F for 30 min.
To make the icing, melt ½ cup of chocolate chips in a heat-safe bowl, then fold in ¼ cup of sour cream.
Once your brownies have been cooled for 20 minutes, spread the icing across the brownies and top with the remaining pistachios.
Serve and enjoy :)
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Notes
Chocolate and butter can be tricky ingredients to melt since they both burn very easily. The best way to melt your chocolate and butter for these delicious sour cream brownies is by using a double boiler. If you don't have one, you can make your own by placing a heat-safe bowl or stainless steel pot on top of a larger pot of simmering water. The heat from the simmering water melt the chocolate and butter without scorching them. Just becareful not to get any water in your chocolate and butter, and stir continuously to ensure even melting. You can also use a microwave for melting your chocolate and butter, only heating for 30 seconds at a time and stirring in between to keep from burning.
To check if the brownies are done, insert a toothpick or skewer into the center. If it comes out with a few moist crumbs clinging to it, the brownies are ready. Avoid over-baking, as this can result in dry brownies.
The best brownies have smooth, chocolatey icing. You can whip up a simple chocolate frosting by melting ¼ cup of semi-sweet or unsweetened chocolate chips and 3 tablespoons of butter in your double boiler. Once melted, mix in 1 teaspoon of vanilla, 2 tablespoons of milk, and 1 - 1 ½ cups of powdered sugar to taste until combined.