This ganache is easy, chocolate loveliness that can be used as frosting or glaze. Made with just bittersweet chocolate, heavy cream, butter, and vanilla—it's a cinch to whip up and lick off a spoon.
In a medium saucepan, heat the cream and butter over low heat until the cream is hot and the butter melts. The hot cream mixture should form tiny bubbles and measure about 175°F (80°C) on an instant-read thermometer; do not let the mixture boil as a skin might form on the top. (If this happens, use a spoon to lift off the skin and discard it.) Remove the pan from the heat, add the chocolate, and let it stand for about 30 seconds until the hot cream mixture softens the chocolate. Whisk the ganache just until all of the chocolate is melted and the ganache is smooth. Stir in the vanilla.
If the ganache will be poured or spread, let it cool and thicken slightly, about 30 minutes. It can be covered and refrigerated for up to 1 week; if it becomes too firm, it can be warmed over low heat, stirring to soften it evenly.