Feed your sourdough starter 2-10 hours before you make this recipe so it will be nice and bubbly. If you don't feed it, it will still be fine, you just won't have grains that are as broken down.
Preheat your oven to 400°F.
Whisk the dry ingredients together in a large mixing bowl: flour, baking powder, baking soda, cinnamon, nutmeg, and salt.
Add the melted butter, brown sugar, white sugar, vanilla extract, milk, and egg in a medium mixing bowl and mix well with a large spoon.
Pour in your sourdough starter with the wet ingredients and mix well with a wooden spoon.
Add your wet ingredients to your dry ingredients and mix until just combined.
Add muffin liners to a muffin tin or spray with non-stick spray. Fill the cups ½ way full and place in a preheated oven for 20 minutes, or until the muffins bounce back when tapped on top.