Chocolate Shot Glasses

Your best bet is to start with a good quality tempered chocolate and melt it just to the point of melting, which is to say keep it 88° - 91°F (if you ever do end up getting a thermometer). Most suggest using a microwave for this in short bursts to keep the chocolate from getting too hot. If you use a double boiler, use a low heat and go slow. Pull the bowl off the heat often and stir. It may not seem like all the chocolate is going to melt, but keep stirring and add small amounts of heat. Or you can follow Alton Brown and sandwich a heating pad between two bowls and adjust the heat.

The theory behind this is there are 4 different types of crystals in chocolate. The good ones we want are Beta crystals, which do not melt until 91° - 94°F. As long as you don't melt the beta crystals, the chocolate should set back up to a proper temper.

Use a grater to make nice even bits, melt the chocolate 2/3 of the way and then remove from the heat. Stir off the heat until the rest melts, or add heat in very short bits, 5-10 seconds at a time. Don't let it harden either, it's harder to remelt if it's hardens up slighty as it then becomes one big solid mass to melt.

Once you break the temper, there is no going back, you have to start over and there is no real easy way to do that without years of experience as bobobobo said, or a thermometer. You can find inexpensive thermometers online, just have to make sure to get one that handles the lower range of temperatures. I use my chocolate thermometer to also measure the temperature of water when making bread. It seems more accurate at the lower temeratures.

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.