Preheat oven to 350°F and line a 12-cup muffin tin with paper liners or spray liners with non-stick spray.
In a large bowl, mix rolled oats, cinnamon, baking powder and salt.
In another bowl, whisk together almond milk and flaxseed. Let sit for about 5 minutes before adding remaining wet ingredients: mashed banana, maple syrup, peanut butter and vanilla.
Pour wet ingredients into the large bowl with the dry ingredients. Gently stir to combine.
Scoop mixture evenly into muffin tin with liners and top each cup with slice of banana, if desired.
Bake 30-35 minutes or until the center of the cups has set and a toothpick comes out clean. Allow cups to cool before transferring to a storage container and placing them in the fridge or freezer.