The way you've described is precisely how I melt chocolate. If you have a double boiler, that's even better, but a bowl on top of a pot is fine too.
I can only think of two things that might be affecting the quality of your end result:
Is the bowl big enough? The melting bowl should be larger than the pot if possible; you want the steam to be forced under and around it.
Is the water temperature reasonable? You want it to be at a simmer. If it's rapidly boiling, the heat is too high.
As long as you keep those two things in mind, your chocolate should melt fine!
Edit: Thought of one other thing:
- It's possible that the steam is actually rising above the bowl, then hitting something (like your range), condensing and then falling back into the bowl as water. You shouldn't even be getting much steam with this method, but just in case, turn your fan on, to make sure you aren't getting any condensation.
Chocolate is a sol, consisting of solid particles suspended in cocoa butter. It is something similar to a hard emulsion. And it can separate just the way a liquid emulsion does (think mayonnaise). This happens when you melt the cocoa butter completely, so the solid particles separate from the fat. If it happens to a chocolate bar, your chocolate looks grey. If it happens to a bowl of melted chocolate, the chocolate seizes the way you describe it. This happens with both milk and dark chocolate. If you haven't experienced it with milk chocolate before, you either had luck, or your milk chocolate was of a lesser quality than the dark one and contained non-cocoa fats and/or emulsifiers, which change the behavior of the sol.
The only way to prevent seizing is to work within the correct temperature zone, which is extremely narrow (2-3°C). Even as an experienced confiseur, it is extremely hard to judge it intuitively. If you insist on trying to watch the chocolate and guess when it is OK, you will have inconsistent results, with a seizing once every few tries.
What you need is to get a candy thermometer. Keep it in the chocolate and, whenever the temperature nears the danger zone, put the inner bain marie vessel in a basin of cold water you keep near the stove for this purpose. It will cool rapidly and stay tempered. It isn't a problem if you cool it off so much it hardens again; you can remelt chocolate as often as you want as long as you never exceed the seizing temerature.
And now for the numbers. All kinds of chocolate (milk, white, bitter) harden at 27°C. Between 27°C and 30°C, they are soft, but unworkable, because they are too viscous (hold unpacked chocolate pieces in your hand for a while to see what I mean). The workable zone is 30°C to 32°C for milk (and white) chocolate, and 30°C to 33°C for dark chocolate. Above this, your cocoa fat melts and the chocolate seizes. So, keep an eye at the thermometer, as you see, the zones are narrow.
Edit: I just noticed that you call 40% "bitter". This is a very low cocoa percentage, and I wouldn't let it go up to 33°C. The numbers for "bitter" are probably safe for 70% cocoa and above.
Best Answer
If there is no heat, the chocolate is not melting; something else is going on. The closest thing I could find to an explanation was on this site about how chocolate is made:
Of course, this is in reference to pure cacao, an ingredient in chocolate making, rather than to a finished product like that egg in the video. Still, it points to a mechanism by which the liquid in the sugar solution might interact with the chocolate in the egg that is neither melting nor dissolving the chocolate, but might make it look like the egg was melting or being dissolved.
I also found some discussion of sugar bloom on chocolate that's exposed to most conditions. The problem there is that the moisture is dissolving the sugar out of the chocolate; in your scenario, the sugar solution is already saturated and the change is taking place very rapidly. I would tend to give more weight to the explanation about the cellulose in the cocoa solids.