Apart from the spelling, they actually are completely different products.
Cocoa Powder is roasted, ground and chemically processed cacao beans, often with sweetener added.
Cacao Powder refers to the raw form of chocolate (including the tree and bean). It is not roasted, processed, and does not contain any sweetener.
Raw cacao powder is very rich in antioxidants and essential fatty acids. It is full of B vitamins for energy production, magnesium for relaxing your body, theobromine (stimulates secretion of happy hormones serotonin, endorphins & dopamine) fibre, iron, copper, zinc, magnesium and sulfur.
However once cacao has been chemically processed, it loses almost all of its nutritional benefits, and becomes cocoa powder. This is what we typically find on our supermarket shelves.
Raw cacao is more of an investment, but it may be worth it for your health.
You can find it in health food stores, as well as in many supermarkets. I add it to my fat-burning coffee in the morning, smoothies, baking, yoghurt.... Just about anything! This is some chocolate you can feel seriously good about.
Let me know if you've tried it and how you found it!