Preheat the oven to 375°F (190°C) and line muffin pans with baking cups.
In a large mixing bowl, combine the flour, sugars, nutmeg, salt and oil.
Take out ½ cup of this mixture and combine in a small mixing bowl with the cinnamon and mini chocolate chips (or other add-in).
Add the sour milk, egg, baking soda and baking powder to the original mixture and whisk together, making sure everything is well-combined.
Fill each baking cup half full. Be careful not to overfill or the middles will sink in!
Top each muffin with 1 to 2 tsp of the cinnamon mixture.
Bake one pan at a time for 10 to 15 minutes, or until the tops are golden brown.
Let the muffins rest in the pans for 5 minutes before transferring to a wire rack to cool completely.
Store in an airtight container. These muffins also freeze well.
