Preheat oven to 350°F (176°F). Line an 8x8-inch pan with parchment paper or grease with nonstick spray.
Add the milk, egg, egg yolk, maple syrup, vanilla extract, and coconut oil or melted butter to a large bowl. Use a whisk to mix until smooth. Add in the oats, baking powder, cinnamon, salt, and mini chocolate chips (see note 4). Stir with a wooden spoon until combined.
Use a spatula to scrape every bit of batter from the bowl into the prepared pan. Using the back of the spatula, smooth in an even layer. If desired, sprinkle coarse sugar, turbinado sugar, or brown sugar on top. Bake for 25–30 minutes or until lightly golden on top and the middle is set. The longer it bakes, the firmer the oatmeal is. Remove sooner for softer baked oatmeal.
Remove and allow to cool slightly. Cut and serve in bowls. If desired, pour a drizzle of milk and maple syrup over the bars.
Notes
Note 1: Discard or save extra egg whites to use in a different recipe.Note 2: Use pure maple syrup, not pancake or corn syrup. The oatmeal is sweet as written, so feel free to reduce the syrup and add more when serving if needed.Note 3: Use old-fashioned oats. Quick oats and steel-cut oats won't work properly.Note 4: For less sweetness, reduce the chocolate chips to 1/2 cup. Regular chocolate chips can be used instead of mini chips.Nutrition Note: The nutrition information does not include the optional sugar topping.Storage: Don't prepare the batter ahead of time. Bake immediately after mixing. Let baked oatmeal cool completely, then store in an airtight container in the fridge for up to 4 days. Reheat individual portions in the microwave and add milk, coffee creamer, or maple syrup if desired.