These muffins can be started 4 to 8 hours before baking or the night before. I usually start them the night before and bake them in the morning.
Begin melting the butter and let it cool slightly before adding it to the mixture. Mix together the flour, room temperature sourdough starter, bananas, sugar, butter, and vanilla. After mixing, cover, and let sit for 4 to 8 hours or overnight.
After fermentation, preheat the oven to 350F.
Add the remaining ingredients. Add the eggs, salt, frozen or fresh blueberries (save about 12 berries to top your muffins with just before baking), baking powder, and baking soda to the fermented batter and mix well.
I used frozen blueberries. That way, when mixing the berries into your batter, they don't become a mushy mess. Using frozen blueberries did not affect my cooking time.
Pour the batter into the prepared muffin tins and add two or three bluenerries on top to get a beautiful bake.
Bake for 20 to 25 minutes or until a toothpick comes out clean of batter. Allow the muffins to cool slightly before enjoying.