Ganache—a smooth mixture of chocolate and cream—is one of the richest, most luscious of all chocolate frostings. This recipe is from "Martha Stewart's Cupcakes."
Place chocolate in a large heatproof bowl. Bring cream and corn syrup just to a simmer over medium-high heat; pour mixture over chocolate. Let stand, without stirring, until chocolate begins to melt.
Beginning near the center and working outward, stir melted chocolate into cream until mixture is combined and smooth (do not overstir).
Refrigerate, stirring every 5 minutes, until frosting just barely begins to hold its shape and is slightly lighter in color. Use immediately (ganache will continue to thicken after you stop stirring).
Achieving the perfect consistency can be tricky; if the frosting becomes too firm to spread, reheat in a bowl over a pan of simmering water until it begins to melt around the edges, then remove from heat and stir until smooth.