In a quart jar or a large airtight container, mix the oats, chia seeds, dates, maple syrup, vanilla, cocoa powder and milk. Stir well with a wooden spoon. If the cocoa powder does not dissolve straight away, leave it for a few minutes, then stir again.
Seal the jar or container tightly and refrigerate overnight (at least 6 hours).
To serve, stir well. Place into bowls and top with berries and, if you like, a small drizzle of maple syrup.
