Combine cocoa, marshmallows, and milk in top of a double boiler. Cook over boiling water, stirring constantly, until marshmallows melt and mixture is well blended. Remove from heat; add vanilla. Cool.
Beat 2 cups whipping cream in a large mixing bowl until soft peaks form. Fold cooled marshmallow mixture into whipped cream; spoon mixture into a 6- cup mold. Freeze overnight.
Unmold onto a serving platter. Garnish with whipped cream and marshmallow hearts, if desired. Sprinkle with grated chocolate and chocolate curls. Let stand 10 minutes before serving. Slice and serve on individual serving plates.