Once the chocolate and butter have fully melted, remove from heat. Whisk the mixture gently to release more heat, before whisking in 1/2 cup crème fraîche and 1/4 cup half-and-half. Continue whisking until both the crème fraîche and half-and-half are fully integrated and the frosting is a uniform dark chocolate color. Set the frosting aside for 15 minutes to cool some more, giving the frosting a gentle whisk or two every 5 minutes to allow heat to escape. After 15 minutes, use the frosting. At first, it will seem too liquidy, but the frosting will quickly cool as it is spread throughout the cake. Work quickly to frost the cake before the frosting cools completely — it will harden as it cools.