- Preheat your oven to 375°F (190°C) and line a muffin tin with paper liners or grease it lightly. 
- In a large mixing bowl, whisk together the flour, sugar, baking powder, baking soda, and salt until well combined. 
- In another bowl, mix the melted butter, buttermilk, eggs, and vanilla extract until smooth. 
- Pour the wet ingredients into the dry ingredients and stir gently until just combined. Be careful not to overmix; a few lumps are okay. 
- Gently fold in the blueberries and lemon zest, if using. 
- Divide the batter evenly among the muffin cups, filling each about 2/3 full. 
- Bake for 18-22 minutes or until a toothpick inserted into the center comes out clean. 
- Allow the muffins to cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.