Combine the oats, chia seeds, milk, espresso, maple syrup and vanilla extract together in a small baking dish. Whisk together until smooth, making sure to break up any chia seed clumps- I suggest using a whisk to mix.
Refrigerate overnight or for at least 4-5 hours, until the mixture has thickened. You may need to mix the oats again after refrigerating to make sure everything is well combined.
Prepare the yogurt topping by mixing the greek yogurt with the milk, maple syrup and vanilla extract. If your yogurt is thick, add milk by the tbsp until the yogurt is smooth and pourable.
Pour the yogurt over the oats and spread evenly. Dust with cocoa powder and serve cold.