Preheat the oven to 425°F (220°C). Grease the muffin pan with non-stick oil spray or line it with paper liners, and set aside. If using paper liners, lightly grease them as well.
In a medium bowl, whisk flour, ground pistachios, baking powder, baking soda, salt, and lemon zest until well combined.
In a large mixing bowl, mix melted butter, sugar, sour cream (or yogurt), milk, lemon juice, and vanilla extract until well combined, and the mixture is smooth.
Fold dry ingredients into wet ingredients and gently mix them until everything is well combined. Do not overmix.
Divide the batter evenly between the muffin tins. Fill each muffin tin all the way up to the top. I recommend using an ice cream scoop for easy and even distribution of the batter.
Bake the muffins for 5 minutes at 425°F (220°C), and then, keeping the muffins in the oven, reduce the oven temperature to 350°F (180°C).
Continue baking for 13-15 minutes or until the cake tester inserted in the center comes out clean. However, every oven is different, so once you reduce the temperature, keep an eye on the oven after 12 minutes of baking.
Remove the muffins from the oven and let them cool in the pan for 5 minutes. Then transfer them onto the cooling rack and let them cool completely.