Preheat oven to 400 degrees F.
In a large bowl, add the flour, salt, baking powder, or baking soda. Mix well.
Add the cranberries and mix into the flour mixture.
In a separate bowl, add the milk, sourdough starter, honey/maple syrup, orange juice and zest, oil, egg, and vanilla. Whisk until thoroughly combined.
Fold in the millet if using.
Pour wet ingredients over dry ingredients and mix until just combined.
Scoop out the batter into a prepared 12 cup muffin tin evenly.
Bake in preheated oven for 15-18 minutes until lightly golden brown.
A toothpick stuck in the middle of a muffin should come out clean.
Cool on a wire rack completely before storing.