In a microwavable bowl or double broiler, combine the sour cream, butter, and chocolate. If using microwave turn down the power a little and stir every 45 seconds or so. Heat and stir just until chocolate is completely melted.
Throw it in the freezer while the cupcakes bake until it's pretty firm. Beat for at least 5 minutes until fluffy. Frost cooled cupcakes immediately and then chill. I suggest serving them cold. That's how they're the best.