01 - Preheat oven to 375°F. Line a 12-cup muffin tin with paper liners or grease thoroughly with butter or cooking spray.
02 - In a large mixing bowl, whisk together all-purpose flour, granulated sugar, salt, baking powder, and baking soda until well combined.
03 - In a separate bowl, whisk sourdough starter, eggs, melted butter, whole milk, and vanilla extract until smooth and fully incorporated.
04 - Pour wet ingredients into dry ingredients. Gently fold together using a spatula until just combined—small lumps are acceptable. Do not overmix as this will create tough muffins.
05 - Fold in fresh or frozen blueberries and lemon zest if using. If using frozen berries, add them directly from the freezer without thawing.
06 - Divide batter evenly among the 12 muffin cups, filling each approximately 3/4 full. Use a cookie scoop or measuring cup for consistent portioning.
07 - Bake for 22-25 minutes until tops are golden brown and a toothpick inserted into the center comes out clean or with just a few moist crumbs.
08 - Allow muffins to cool in the pan for 5 minutes before transferring to a wire rack. Cool completely before storing in an airtight container.