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.