Preheat the oven to 375°F and line a muffin tin with paper liners or grease it well.
In a large bowl, whisk together the melted butter, sugar, eggs, vanilla, milk, and sourdough starter until smooth and well combined.
In a separate bowl, combine the flour, baking soda, baking powder, salt, and cinnamon. Stir well.
Slowly mix the dry ingredients into the wet ingredients, stirring gently until just combined. Avoid overmixing to keep the muffins light and fluffy!
Coat the diced strawberries with 1 tablespoon of flour to help keep them evenly distributed in the muffins. Gently fold the coated strawberries into the batter.
Spoon the batter into the muffin cups, filling each about ¾ full. Bake for 18-22 minutes, or until a toothpick inserted in the center comes out clean.