1) Place the cream, marshmallows, confectioners' sugar, and salt in a large, deep saucepan.
2) Heat over medium-low heat, stirring frequently, until the marshmallows melt completely.
3) Remove from the heat, and stir in the vanilla, if you're using it.
4) Pour the icing into the bowl of your stand mixer or another mixing bowl, and refrigerate until thoroughly chilled, preferably overnight.
5) Remove the icing from the refrigerator. Beat it at high speed, using your stand mixer or an electric hand mixer; it's impossible to beat thoroughly enough by hand, so please use a machine. Scrape the sides and bottom of the bowl to make sure you're incorporating all of the chilled icing.
6) Beat until the icing is smooth and glossy; this will probably take less than a minute. The icing will thin out and become almost sauce-like. Once it's smooth, set the icing aside to rest at room temperature for 30 to 45 minutes; it'll stiffen as it sits, bringing it to spreadable consistency.
7) Spread or pipe the icing atop cupcakes, or use it to fill a round layer cake, or ice a 9" x 13" sheet cake.
Yield: 3 to 4 cups icing, enough to ice 24 cupcakes, one 8" or 9" round layer cake, one 9" x 13" cake, or a typical recipe of whoopie pies (about 12 to 18 pies, depending on size).