Fill a large metal bowl with equal parts ice and water. Place a smaller metal bowl over the ice to create a water bath.

Place the water and chocolate in a medium saucepan. Heat over medium-low heat, whisking until the chocolate has melted.

Pour the chocolate into the bowl over the water bath. Whisk constantly by hand until the chocolate thickens into a mousse.
If the mixture becomes grainy, you've whisked it a little too much. Simply melt the chocolate again and whisk.
If the chocolate mousse doesn't come together into a mousse, you need more cocoa fat. Just melt the chocolate again, add a little more chocolate and whip it one more time.
If the mousse is too dense and not light enough, melt the chocolate again and add a little more water. Return the chocolate to the water bath and whisk until the mousse is perfect.

Spoon the mousse into jars or dessert dishes. Serve immediately or refrigerate for up to 5 days.

Serve with whipped cream and or berries if desired.