1/2 cup cream
1 tablespoon unsalted butter
2 teaspoons vanilla extract
For the ganache, place the chocolate in a stainless steel bowl. Heat the cream and butter in a saucepan until just boiling, then pour over the chocolate. (Note: Don't take the cream off the stove too early - make sure it definitely is boiling or the chocolate won't melt.) Whisk until smooth. Add the vanilla extract. Let stand until cool but still smooth and glossy.
Source:
http://www.whatmegansmaking.com/2010/06/homemade-hostess-cupcakes.html