How do I make sugarfree chocolate mousse made with 100% cacao chocolate less stiff? I recently made chocolate mousse with 50g of 100% chocolate, and 2 egg whites. No sugar, sweeteners, or cream added. This resulted in a very stiff and even slightly grainy (but that might just be because my chocolate wasn't hot enough) chocolate mousse. Trying the same recipe with 70% chocolate results in a slightly softer, consistent mousse that is easier to handle and mix during the preparation. It becomes even easier to make when a bit of sugar is added.

The sugar seems to alter the texture of the chocolate mousse to make it somewhat fluffier. How do  i replicate the texture of a 70% cacao chocolate mousse when using 100% cacao chocolate without adding sugar? If possible I'd like to make a chocolate mousse without adding cream because I like the lighter feeling of a mousse made with just egg white + chocolate. Do I just need less chocolate in relation to the egg white to make up for the increased concentration of cacao so that the 70% and 100% cacao mousse end up with the same amount of cacao in them rather than the same amount of chocolate?
So there are a few things you could change here. 

Firstly I would use 85g of dark chocolate for every 2 egg whites. 
When melting your chocolate, 2 tablespoons of hot water will help smooth it out stopping that graininess and make it less stiff/easier for the egg white stage. 
Leave to cool and whip up your egg whites. Then fold in a third of the egg whites, then fold in the rest. 

If you don't want to use cream, I would recommend trying 50g of greek yoghurt into the cooled chocolate before adding the egg whites. This is not as heavy as cream, will keep the smoothness of the chocolate and also keep it light and fluffy.