Whisk together the flour, baking powder, baking soda, cinnamon, nutmeg and salt in a large mixing bowl. Add melted butter, brown sugar, white sugar, vanilla extract, milk and egg. Mix well with a spoon. Pour in sourdough starter and mix with spoon.
Spray a large six cup muffin tin or a 12 cup muffin tin with cooking spray. Fill cups 1/2 full. Bake in a preheated 400 degree oven for 17 to 20 minutes.
Makes 6 large muffins or 12 regular muffins
Notes
I add a pinch of cinnamon and sugar to tops of muffins before I cook them but this is optional.