Preheat your oven to 400°F (200°C). Line a muffin tin with paper liners.
In a large bowl, whisk together flour, sugar, baking soda, and salt.
In a separate bowl, combine vinegar and milk. Let it sit for a minute to fizz.
Add the wet ingredients to the dry ingredients, along with the egg and melted butter. Stir gently until just combined. Don't overmix.
Fold in the blueberries (or your favorite mix-ins).
Fill the muffin cups about 2/3 full.
Bake for 18-20 minutes, or until a toothpick inserted into the center comes out clean.