Preheat oven to 425°F. Line a 12-cup muffin tin with cupcake liners. Combine the oats and milk in a large bowl and let rest at room temperature for at least 10 minutes.
Meanwhile, combine the flour, baking powder, cinnamon, baking soda, and salt in a separate bowl.
Whisk the sour cream, oil, and brown sugar into the bowl with the oat mixture until smooth.
Whisk in the eggs, one at a time, until smooth. Whisk in the vanilla.
Gently fold the dry ingredients into the wet ingredients, mixing only until just combined. Be careful not to overmix here! Fold in the chocolate chips.
Divide the muffin batter between prepared muffin cups, filling each about 2/3-full. Sprinkle with additional oats and chocolate chips, if desired.
Bake 5 minutes at 425°F, lower the temperature to 350°F and bake for an additional 18-22 minutes, or until the centers are set and spring back slightly when touched gently.
Allow the muffins to cool completely in the pans before transferring to a wire rack. Store muffins in an airtight container in the refrigerator for up to 1 week.