At least 12 hours before you wish to bake them, combine sourdough starter and flour. It will make a very thick dough, so do not be alarmed. Cover and place in a warm spot to culture for 12 to 24 hours.
After 12 to 24 hours, preheat oven to 375°F. Combine sugar, banana, and egg in a small bowl. In another small bowl combine salt, baking soda, baking powder, and cinnamon.
Sprinkle the dry ingredients over the cultured dough. Gradually add the liquid ingredients, stirring just to combine.
Spoon batter into muffin tin, filling 3/4 full. Bake at 375°F for 18 to 20 minutes, or until a toothpick comes out clean.