Preheat the oven to 425°F (220°C). Line the 12-cup muffin pan with paper liners.
In a medium bowl, whisk flour, baking powder, baking soda, and salt together until well combined.
Place the sugar and lemon zest in a bowl. Rub the two between your hands, working the zest into the sugar until fragrant.
Remove two tablespoons of lemon sugar and set aside for the muffin topping.
In a large mixing bowl, combine ricotta cheese, sour cream, melted butter, milk, and vanilla. Whisk well until everything is well combined and smooth.
Add lemon sugar and mix until well incorporated.
Fold the dry ingredients into the wet ingredients, then gently mix until everything is well combined. Do not overmix; the batter will be thick.
Divide the batter evenly between the muffin tins. Fill each muffin tin all the way up to the top. Sprinkle the reserved lemon sugar over each muffin.
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 14-16 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.