Chocolate Storage – Why Does Dark Chocolate Turn White?

From the recipe and your "fudge" description, I think the problem was that you didn't get enough air in the batter. Trying a different recipe is a good idea.

The way to get more air into a flourless cake is through the eggs. Look for recipes which require you to beat the egg whites separately to soft peaks. Mix everything very carefully, in the correct order. If your try still doesn't get you enough air, try more tricks. Whip your yolks with some liquid (maybe 10 ml per yolk) over a water bath (keep it between 60° and 70°C!), practically creating zabaglione. But really watch the temp with a probe thermometer - you don't get a good emulsion below 60°C, and you don't want your proteins to coagulate while whipping, they must do it latter in the oven (they start a bit above 70°C).

The other problem is that you might get too smooth a texture. First, select a recipe with less fat. Fat makes the batter softer, but also very smooth. Your hunch is correct: Choose a recipe which calls for both cocoa powder and chocolate. The powder has much less fat, resulting in a higher percentage of starch, and that creates a more cakelike texture. Another good way to change the texture is to use recipe which includes a nut flour. A raw almond flour will give you very little change in flavor, but if you don't mind deviating from the original, roasted hazelnut pairs very well with both chocolate and raspberry.

And as we are speaking of fat: you must use a good chocolate! Standard chocolate bars (Hershey, Milka) are no good. To cut costs, they use vegetable fat instead of cocoa butter, which changes the texture a lot. Then there are the manufacturers who use emusifiers and include even less cocoa butter. If you can get hold of real confectioner's chocolate (Valrhona, Callebaut), use it. If not, buy a chocolate bar which contains only cocoa mass, cocoa butter and sugar and nothing else - no vegetable fat, no emusifiers, etc. Also no milk solids, you want to make this kind of recipe with bittersweet to bitter chocolate. The one big brand I use for baking is Lindt Excellence (not Lindt Lindor!), but there are also some supermarket-branded chocolates with the right contents.

I don't have a tried recipe to offer you, but I am sure that the usual suspects have published something good. I think Smitten Kitchen had a nice big post on flourless pastries ones (but don't remember if they had a choco cake there), and Lebovitz and/or Desaulniers probably have recipes for a flourless choco cake too.

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.