Mash the banana: In a jar or bowl, mash half a banana until smooth.
Add wet ingredients: Stir in almond milk, vanilla, and sweetener if using.
Mix in protein: Whisk in the protein powder until no lumps remain. Add a splash more milk if it gets too thick.
Add the rest: Stir in oats, chia or flax, cinnamon, and a pinch of salt.
Swirl in peanut butter: Dollop the peanut butter and gently swirl it through the mixture.
Chill: Cover and refrigerate for at least 4 hours, ideally overnight.
Serve: In the morning, stir, adjust thickness with a bit more milk, and top with a few banana slices if desired.