Preheat oven to 350FoF. Spray a muffin pan with nonstick cooking spray.
Melt butter in the microwave and set aside.
Chop pistachios into small pieces.
In the mixing bowl, sift flour, baking powder, baking soda and salt. Set aside.
In the large mixing bowl, whisk together sugar and eggs. Add pistachio paste and sour cream. Whisk until all combined.
Add the dry ingredients and chopped pistachios. Leave some pistachios for topping later. Mix with spatula until evenly combined.
Using ice cream spoon or regular spoon, fill the muffin cups about three-quarters full. Sprinkle the top with chopped pistachios.
Bake muffins for 18-20 minutes. Let the muffins cool in the pan for 5-10 minutes, then transfer to a wire rack to cool completely.