Melt the chocolate in the top half of a double boiler. While the chocolate is melting, combine the butter and sugar in the bowl of an electric mixer. Using the paddle attachment, cream on medium speed until the mixture is light and fluffy. Switch to the whisk attachment and add the eggs 2 at a time, mixing well after each addition. Scrape the sides of the bowl, increase to medium-high speed, and whip for 2 minutes, until the egg mixture increases slightly in volume.