Add the rolled oats, cocoa powder, protein powder, and chia seeds to a bowl or jar, then stir thoroughly so the dry ingredients are evenly mixed and the cocoa and protein powder are well distributed without clumps.
Add the Greek yogurt, milk, and sweetener to the bowl, then mix slowly and steadily until everything is fully incorporated and the mixture looks smooth, creamy, and evenly hydrated.
Cover the container tightly with a lid or wrap and place it in the refrigerator for at least 4 hours or overnight, allowing the oats and chia seeds to absorb the liquid and soften while the flavors develop.
Before serving, remove the oats from the refrigerator and stir well to loosen the texture, adding a small splash of milk if needed until the oats are thick, creamy, and easily spoonable.
Add your desired toppings, such as fresh fruit, nuts, or seeds, and enjoy the overnight oats chilled for a satisfying, high-protein breakfast or snack.