In a large mixing bowl, combine the rolled oats, peanut butter, honey (or maple syrup), salt, and vanilla extract. Stir until well combined and the mixture is sticky.
If using chocolate chips, fold them into the mixture.
Line a muffin tin with paper liners or lightly grease it.
Divide the mixture evenly among the muffin cups, pressing down firmly to pack it in.
Refrigerate the cups for at least 30 minutes to set.
Once set, remove the cups from the muffin tin and enjoy! Store any leftovers in an airtight container in the refrigerator.