How to keep a chocolate cream from getting hard? I've been making a chocolate cream/spread lately. Some butter, chocolate, salt, allspice, cocoa and water. I'm not really using any recipe, but it's about twice the chocolate over butter.

These ingredients are mixed at a low temp, the water gets added after the rest is melted and stirred together. After cooling, this mixture stays usable at room temperature for some days. That is, it can be spread on bread. But after some days, the mixture becomes brittle and hard.

I suppose this is because water evaporates out. Does anybody know how this can be prevented or reverted? Should I melt it and mix more water in? Is that possible?
I had to do some reading on this one. There are a lot of answers.

1- You should find a better recipe. Chocolate spread recipes I have used come in one of two styles. 

The water in your recipe seems to be a problem. The nutella style that uses fat, sugar, and chocolate. These will not really dry out. Sometimes the oil will separate a little and will need to be mixed back in. I would group frostings in this group although, of course, they contain far more sugar.
See this recipe:
http://www.foodnetwork.com/recipes/gale-gand/chocolate-hazelnut-spread-with-crepes-recipe/index.html

Thick chocolate candy. AKA fudge. These contain water but the sugar and water are cooked together into a syrup so the water is not able to immediately dry out. They will crystallize if they are badly handled- many recipes contain a little corn syrup to prevent this. This does not seem to be what you are trying to make.

2- Store your concoction in a lidded container that isn't too much larger than your sauce. I use mason jars to store mine. Alternatively you can push plastic wrap down onto the surface of the spread. 

3- Refrigeration. Recipes that don't contain dairy don't need to be refrigerated and in fact shouldn't be because it causes them to harden.

4- Adding water to melted chocolate is only a problem when dealing with chocolate that is dry and meant to solidify such as a molded chocolate. In such a case even a drop of water will cause seizing. In this case where there is already quite a bit of water in the recipe seizing is not a risk and you shouldn't have any problem mixing a tiny bit back in. Do not add water to the nutella-like spread recipes. They do not contain water and it will not mix in.

I can't tell from your description what is causing your sauce to harden- it could be drying out, separating, or crystallizing. Any of these will be solved by reheating and thoroughly stirring. Although water could be added to restore dried syrup I think you will get better results by switching to a recipe that uses fat in place of the water.