01 - Preheat the oven to 375°F. Line a 12-cup muffin tin with paper liners or grease thoroughly.
02 - In a large bowl, whisk together flour, sugar, salt, baking powder, and baking soda until well blended.
03 - In a separate bowl, whisk sourdough starter, eggs, melted butter, milk, and vanilla extract until smooth and combined.
04 - Pour wet ingredients into dry ingredients. Gently fold together until just combined. Do not overmix—a few lumps are fine.
05 - Fold in blueberries and lemon zest if using, being careful not to crush the berries.
06 - Divide batter evenly among muffin cups, filling each about 3/4 full.
07 - Bake for 22–25 minutes, or until a toothpick inserted into the center comes out clean and tops are golden brown.
08 - Let muffins cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.