Chocolate seized during dipping, how to prevent it

In order to get the chocolate to harden correctly, still look shiny, and have that nice snap when it is broken, you need to temper your chocolate. There are many methods for doing this, but the seeding method on this site is most people's preference:

http://www.cookingforengineers.com/article/155/Tempering-Chocolate

After that you tempered chocolate, follow this process:

• Ensure molds are 100% dry
• Fill molds completely with melted chocolate.
• Tap the molds on the table or tap them with a spoon for a few seconds to get rid of the air bubbles.
• Invert the molds over your bowl of chocolate and let them drain out, leaving a chocolate coating in the mold.
• Place the molds face down over a sheet of parchment paper (or something like that) to let them drain out a little more.
• Put them right side up again and let them dry for 20-30 minutes.
• (optional) place the molds in the fridge for 10 minutes (will make demolding eaiser)
• Invert the molds over a sheet of parchment paper. If necessary, tap gently or carefully twist the mold to get the pieces to release.

EDIT: First, if you are going with this technique, use real chocolate (the only fat should be cocoa butter). Second, if you can, use a high cocoa butter chocolate.

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.