Preheat oven to 375 degrees. In a medium bowl, mix together the oats, baking powder, cinnamon, and salt.
In a large measuring cup, whisk together the milk, honey, eggs, vanilla. Add liquid mixture to dry mixture in the bowl and stir to just combine. Drizzle in butter or coconut oil while stirring, then add mashed banana, blueberries, and nuts.
Spray 12 muffin tins with cooking spray. Distribute batter evenly into tins and bake for 17-20 minutes, or until the baked oatmeal is just starting to brown or a toothpick inserted in the center comes out mostly clean.
Let cool for at least 10-15 minutes before removing from pan and then serve. Refrigerate leftovers in a storage container for up to 5 days, reheating in microwave or toaster oven before serving.