I am sorry, but the accepted answer is incorrect in many details.
When chocolate seizes, it is due to a small amount of moisture. Imagine a cup of sugar. It will pour freely. If you add a small amount of water, clumps of the sugar will stick together and stop flowing. Add enough water, and the combination of sugar and water dissolve together, and flow freely again.
With chocolate, it is the solid particles embedded in the fat phase that interact with water to seize. Adding enough water (about 20% by weight) will un-seize the mixture although it will of course contain much more water.
Similarly, seized chocolate can be recovered for some purposes simply by adding additional water.
This is in contrast to scorching where the chocolate and its components essentially burn, at least locally. This creates a similar texture to seized chocolate, and a terrible flavor. There is no way to recover scorched chocolate.
Of the three reasons enumerated in the original accepted answer, only one is a true cause of seizing:
Using too high a heat. The danger here is scorching the chocolate, not seizing it. You do want to use low and gentle heat, as from a bain marie, double boiler, or simply in the microwave at low power, but this has nothing to do with seizing.
Sugar bloom and other impurities. Neither sugar bloom nor fat bloom cause seizing in any way. In fact, bloomed chocolate can be melted down and then re-tempered, and assuming it has not gone rancid or spoiled, it will be as good as a fresh bar.
Impurities on the other hand, well, it would depend on what they are. Chocolate chips, especially from supermarket brands, often have some of the cocoa butter of true genuine chocolate removed and substituted with cheaper lipids like hydrogenated vegetable oil. These fats do not have the same mouth feel and melting characteristics, but again, are unlikely to change seizing behavior.
Contact with moisture. A small amount will cause seizing. However, it is trivial to recover--add more liquid until the mixture smooths out again. Note that this is again in contrast to scorching, which is not fixable.
So of the first three gotchas, really only the third could cause seizing.
So the most likely culprit for the original poster is low quality chocolate chips, or too high a heat leading to scorching. It is also possible but unlikely that the heat was too high, so that some of the the water boiled out of the butter, leading to seizing—but if that were true, you almost certainly would have scorched the chocolate as well.
Most recipes which direct the cook to melt chocolate and butter together actually have more butter than chocolate, and butter is about 20% water. This is enough that the fully melted mixture should be passed the seizing point.
Using margarine is also perfectly acceptable in terms of seizing. It, like butter, is about 20% water, and so performs similarly. However, if there were some margerine-like product which has a much lower but non-zero water percentage, there might be some danger of seizing.
Furthermore, when melting butter and chocolate together, it is not necessary to pre-melt the butter for two main reasons (I have done this enumerable times, both stove top and in a microwave):
- Butter melts at a lower temperature than chocolate, and so will tend to melt first anyway
- Assuming the heat level is low, and you stir occasionally, the mixture will not scorch, and so will un-seize when everything is fully melted and stirred together.
Of the advise listed in the original, I would clarify:
Using is margarine is fine, but don't use a product that doesn't contain 20% water, equivalent to butter (or one that contains more water).
There is no need to melt the chocolate and butter separately. Clarifying the butter first is actually counter-productive, as it will reduce the water percentage, but probably not down to zero. This may leave enough water to seize, but not to move past the seizing.
Use the best quality chocolate you can find. This is true, but mostly for reasons of taste, in my opinion.
Melt in a double-boiler or on very low heat. A microwave on low power, with occasional stirring, can also be very effective.
I would add that any recipe where butter and chocolate are melted together should have more butter than chocolate, in general, because of the seizing problem. Given that the total water needs to be at least 20% of the chocolate weight, you would need at least equal amounts of butter and chocolate, and preferably double the butter to chocolate (by weight).
A typical brownie recipe that I have made many times, for example, melts two sticks of butter (8 ounces) with 4 ounces of chocolate. That is 3.2 tablespoons of water from the butter, which is enough top prevent seizing.
One final note: melting chocolate together with a pure fat (such as pure hydrogenated vegetable oil) that has no water content is fine at any ratio.
Coda on the science:
Chocolate is normally a solid suspension of solid particles in a fat phase, with only a minuscule percentage of water. So it is water in fat emulsion. Add enough water—about 20% by weight—and the emulsion will switch to being a fat in water (or sugar syrup, as the sugar in the chocolate will tend to dissolve) emulsion, with suspended solid particles, again flowing freely. See Food Education's chocolate article for more details on the science.