In a mason jar or airtight container, combine the oats, milk, yogurt, chia seeds, cocoa powder, and honey. Stir well.
Cover and refrigerate overnight (or at least 4 hours) to allow the oats to absorb the liquid and soften.
In the morning, give it a good stir, add more milk if needed for desired consistency, and top with Greek yogurt if using.
Top with raspberries and dark chocolate chunks.
Enjoy!