Line a muffin pan with paper lines and spray with nonstick spray. (Note 1)
In a large bowl, combine the rolled oats and milk. Let them soak for 10-15 minutes until the oats have absorbed the liquid. I like to melt and cool the butter at this time.
While they're soaking, whisk together the flour, baking powder, baking soda, and salt in a separate bowl.
In a third medium bowl, whisk together the melted butter, brown sugar, eggs, and vanilla extract until well combined. Add the soaked oats to the wet ingredients and mix well.
Gradually add the dry ingredients to the wet ingredients, stirring until just combined. Be careful not to overmix. Gently fold in the chocolate chips into the batter. Let the batter rest for 15 minutes. During this time, preheat your oven to 425°F.
Spoon the batter into the prepared muffin cups, filling each all the way to the top.
Bake in the preheated 425°F oven for 5 minutes then turn the heat down to 350°F, keeping the muffins in the oven, and continue baking fro for 13-15 minutes or until a toothpick inserted into the center comes out clean or with a few moist crumbs.
Allow the muffins to cool in the tin for 15 minutes, then transfer them to a wire rack to cool completely. Repeat with the second batch, turning the temperature back up to 425ºF before baking again.