Preheat the oven to 425° F.
Grease the muffin pan with non-stick oil spray or line with paper liners, and set aside. If using paper liners, lightly grease them as well.
In a large bowl, combine sugar and lemon zest. Then, working with your hands, rub the zest into the sugar until well combined.
Add flour, baking powder, baking soda, salt, and poppy seeds to this sugar mixture. Whisk until everything is well combined.
In a separate bowl, take melted butter. Add sour cream or yogurt, milk, lemon juice, and vanilla. Stir well until everything is combined and the mixture is smooth.
Pour wet mixture into dry and, using a spatula, mix until just 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 (218°C), and then, keeping the muffins in the oven, reduce the oven temperature to 350°F (177°C).
Continue baking for 15-17 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 time.
Remove the muffins from the oven and let them cool in the pan for 5-6 minutes. Then transfer them onto a cooling rack and let them cool completely before serving.