In a large mixing bowl, whisk together the cooled coffee and cocoa powder until the cocoa is completely dissolved with no lumps remaining.
Add the milk, maple syrup, vanilla extract, and sea salt to the coffee mixture. Whisk until everything is evenly combined and smooth.
Stir in the rolled oats and chia seeds, mixing thoroughly so every oat is coated in the mocha mixture.
Divide the mixture evenly among 4 mason jars or airtight containers, leaving about 1 inch of space at the top for expansion.
Seal the containers tightly and refrigerate for at least 6 hours, or preferably overnight for the best texture.
Before serving, give the oats a good stir to redistribute any settled ingredients. Add your favorite toppings like chocolate chips, banana slices, or a dollop of whipped cream.
Enjoy cold directly from the fridge, or warm in the microwave for 60-90 seconds if you prefer hot oats.