Add the rolled oats, milk, peanut butter, cocoa powder, vanilla extract, maple syrup (or honey), sea salt, and chia seeds (if using) to a small jar or bowl.
Stir thoroughly until all ingredients are evenly combined and the peanut butter is dispersed.
Cover the jar or bowl and refrigerate for at least 4 hours or overnight to thicken.
Before serving, stir and adjust sweetness or consistency if desired; top with optional cocoa nibs, extra peanut butter, dark chocolate chips, or sliced banana.