2 ounces unsweetened chocolate, grated or finely chopped
1 cup sugar
3 tablespoons cornstarch
1 tablespoon butter
1 teaspoon vanilla
1/8 teaspoon salt
In a heavy bottomed saucepan, blend sugar and cornstarch; stir in chocolate. Add 3/4 cup of boiling water. Stir. Bring to a boil over medium heat, stirring constantly. Remove from heat and stir in butter, vanilla and salt. Cool completely. If necessary, thin with a small amount of hot water.
Source: The Fanny Farmer Cookbook