If you don’t want the chocolate to thicken, leave it in the bain-marie while you do other things. Just make sure that the bottom of the bowl with chocolate does not touch the water in any way. With excessive heating, the chocolate will stratify, harden, lump. Such chocolate can be saved: add butter and mix well, but it will no longer work as icing. If you see that the melted chocolate is too thick, add a piece of butter or a few tablespoons of warm cream, and mix well. Please, note that you should not melt the chocolate straight from the refrigerator. It should be at room temperature.