It really couldn't be more simple, or more delicious. I adapted Mark Bittman's Chocolate Ganache recipe (below). You can see the video here.
7/8 cup heavy cream 8 ounces bittersweet chocolate, chopped
Heat cream in a pot (or slowly in microwave) until it steams. Put chocolate in a bowl, pour hot cream on top, and stir until chocolate is melted and incorporated into cream. Let cool until it thickens a bit. Pour over your cake and let cool in refrigerator.