center comes out dry. Unmold the cake onto a rack and let it cool; it will fall to about half its original height. Meanwhile, in a double boiler, melt the remaining 3 ounces of chocolate with the remaining 1 teaspoon butter. In a small heavy saucepan, bring the reduced cream to a boil, then whisk it vigorously into the chocolate. Continue whisking until smooth and glossy. Place sheets of waxed paper under the cake and cooling rack. Pour the frosting over the top of the cake, using a metal spatula to spread the frosting evenly over the top and sides. After the frosting has hardened slightly, about 5 minutes, use a long straight knife to mark the top in a crisscross pattern. Mix the cocoa with the remaining 1 tablespoon sugar and sprinkle it decoratively over the cake before serving.