Whether a chocolate is called unsweetened, bittersweet, or semisweet depends mostly on the percentage of cacao the chocolate contains. As the cacao percentage increases, the chocolate itself will taste more intensely chocolatey and less sweet. Bittersweet chocolate generally contains less sugar than semisweet and is called for in recipes that are looking for deep, intense chocolate flavor. But the distinction between the two types becomes hazy between brands.
Semisweet chocolate can usually be substituted with success.
Today, supermarkets and specialty shops offer semisweet and bittersweet chocolates that range from 54% to more than 70% cacao. The choice is exciting, but chocolates with radically different cacao percentages can produce radically different results. Unless a recipe specifies a very high-percentage chocolate, stick with bittersweet chocolate in the 54 to 60% cacao range.
While most recipes call for a double boiler for melting chocolate, a wide, shallow skillet of water with a stainless-steel bowl of chocolate sitting directly in it can work better. The open bath lets you see and adjust the water if it begins to boil or simmer too actively, whereas the water in a double boiler is usually out of sight and thus trickier to monitor. Just as chocolate in a double boiler will scorch if the cook is inattentive, chocolate in an open bath must also be watched carefully, stirred frequently, and removed from the bath when melted.
Well-wrapped and stored in a cool, dry place, dark chocolate has an indefinite shelf life. However, its high fat content means it can easily pick up other flavors, so be careful of what you store near it.