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.
The original author is in error adding baking soda (bicarbonate), and indicating it will help rise. For leavening, you require acid for the sodium bicarbonate to react with. The original base recipe has no significant acid ingredients; even the modified version you used has only a trivial amount of acid from the molasses in the brown sugar.
Therefore, the only effect of the baking soda is to increase the pH of the dough, which will encourage browning. However, as the cookies are full of sugar, encouraging browning is not generally a problem.
The creaming method creates bubbles in the solid phase butter, and encourages rising and a more cake-like result, especially when paired with an effective leavening agent.
Resting the dough also hydrates the flour, and allows the butter to re-solidify, both of which inhibit spread and contribute to a higher cookie.
For a chewier result, do not use the creaming method. Melt the butter instead, and then combine your ingredients.
You may also choose to reduce the baking soda to reduce the metallic taste it brings without anything to react with.
If the cookies are now too dense, you might try adding (starting with smaller amounts) 1/2 tsp to 1 tsp or so of baking powder which has its own acid to react with, to help leaven the cookies.
See also:
Best Answer
Short of re-melting/reheating them you can't do this without some mechanical separation such as cutting between each cookie. You may find that you can do this most easily with a heated knife - dip it in hot/boiling water for a few seconds to heat the blade.