Combine the chocolates, heavy cream, butter and vanilla in a microwave-safe bowl. Microwave 1 minute, then stir. Continue microwaving in 30-second intervals, stirring after each, until the chocolate is just melted. Whisk until smooth and glossy.
Transfer to a fondue pot and keep warm over low heat. Serve with pound cake.
Photograph by Andrew Purcell
Courtesy of Food Network Magazine