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Prick sweet potato with a fork and wrap in foil. Bake at 400ºF until soft, 45 to 60 minutes. Let cool, then remove skin and mash potato flesh. In a jar with a tight-fitting lid, combine milk, chia seeds, cinnamon and vanilla. Cover and shake to combine; refrigerate until thick, at least 4 hours or overnight. Layer chia mixture and sweet potato flesh into 4 cups and drizzle with pure maple syrup, optional.