Preheat oven to 375 °F (190 °C). Line a 12‑cup muffin tin with paper liners or grease well.

In a small bowl, combine oats and milk; let sit for 10 minutes to soften.

In a large bowl, whisk together flour, baking powder, baking soda, salt, and cinnamon.

In a separate medium bowl, mix mashed bananas, sugar, melted butter, eggs, and vanilla until smooth.

Add soaked oats mix to the banana mixture and stir until combined.

Pour wet ingredients into dry ingredients and gently fold just until moistened; avoid overmixing.

If using, fold in optional mix-ins.

Divide batter evenly among the 12 muffin cups.

Bake for 18–22 minutes, or until a toothpick inserted into the center comes out clean.

Cool muffins in the pan for 5 minutes, then transfer to a wire rack to cool completely.