I melted once a chocolate bar and decorated with it my eclairs, while it was hot and melting, after putting them in the refregirator, they cooled fine, but they were a little bit sugary although the eclair's dough didn't contain any sugar.
If I melted a Dark chocolate, will adding some sugar work? Or is it preferred to melt the kind I want?
Melt the kind you want. Sugar doesn't melt at the temperature chocolate melts, and wouldn't dissolve in chocolate anyway. Commercial chocolate bars are smooth because they are conched. If you mix chocolate with sugar, it will be sandy.
Alternatively, you can glaze with a ganache, and dissolve the sugar in the cream first. But it is still better to make the ganache with the type of chocolate you want, instead of adjusting sugar or fat.
The most direct and easiest way would be to use the kind of chocolate you like to eat, that way you don't have to fuss with it too much.
If you do want to adjust the sweetness level to taste, try putting in a touch of a liquid sweetener - honey, agave, corn syrup, etc.
