Unsweetened chocolate and bittersweet chocolate are both dark chocolates. Unsweetened chocolate (also called baking chocolate) is 99 percent cocoa liquor (which is part cocoa butter and part pure cocoa). Bittersweet chocolate, by FDA regulations, must contain at least 35 percent cocoa liquor--but it can contain as much as 84 percent. Because the difference is largely made up by sugar, this creates a product of widely varying sweetness that goes by the same name.
Fill the bottom of the double boiler about halfway with water and place on high heat. Bring to a boil.
Reduce heat to medium or medium low to bring water to a steady simmer.
Break or cut chocolate into chunks or squares. Place upper pot inside lower pot, on top of the boiling water. Add unsweetened chocolate.
Stir chocolate frequently to encourage smooth melting.
Add 1/2 tsp. to 1 1/4 tsp. sugar per ounce of unsweetened chocolate, depending on how sweet you want your bittersweet chocolate to be. Stir frequently to ensure even distribution of sugar. You should also add any additional spices at this time.
If a hard chocolate is desired, pour semisweet chocolate into a mold to harden. Otherwise it is ready for use. Chocolate will harden quickly after it is removed from heat.