What caused my homemade "Nutella" to go bad?

I had a first try on a self-made recipe for a chocolate-hazelnut cream, in short Nutella.

Question Even though I used ingredients that wouldn't have gone bad singularly, the cream as a whole had an intolerable acid smell after 1 week from the preparation. What could have been the cause?

How the cream was prepared I prepared a chocolate-mousse only using 70% dark chocolate and water, then I blended it together with brown sugar, hazelnuts and olive oil. Immediately after preparation the cream was fine. Then I left it on a shelf, and when I came back after 1 week it had a strongly pungent acid smell, the taste was in line with the smell; the situation was bad enough to force me to throw everything away.

Thank you very much for your interest!