Heat oven to 350°F (175°C). Line a 12-cup muffin tin with liners or grease well. Pro tip: silicone liners make release painless.
In a large bowl, whisk rolled oats, quick oats, flax/chia, baking powder, cinnamon, and salt until combined.
In another bowl, whisk milk, eggs, mashed banana, maple syrup, vanilla, and melted oil until smooth.
Pour wet into dry and stir until evenly moistened. It should look pourable but thick—like chunky pancake batter.
Fold in chosen add-ins. If using multiple flavors, divide batter into separate bowls and customize each.
Scoop batter evenly into 12 cups, filling to the top. Press in a few extra berries or nuts on top for the “I'm a pro baker” look.
22–26 minutes, until the centers are set and edges are lightly golden. A toothpick should come out with moist crumbs, not wet batter.
Let rest in the pan 5 minutes, then transfer to a rack. This helps them firm up and prevents soggy bottoms (tragic, IMO).
Enjoy warm or cool completely for storage. Add a swipe of nut butter if you're extra.