Spoon 1 tablespoon of each of the sauces into each of 4 shallow serving dishes, placing them next to each other.
Place a scoop of the chocolate ice cream on the marshmallow sauce and a scoop of the vanilla on the chocolate sauce. Top each scoop of chocolate ice cream with marshmallow sauce and each scoop of vanilla with chocolate sauce.
Add a dollop of the whipped cream to each scoop and serve immediately.
Melt the chocolate with the cream, 6 tablespoons water, and the salt in a heatproof bowl set over a saucepan of simmering water, stirring until smooth.
Use immediately, or let cool to room temperature, transfer to a glass jar, and refrigerate, tightly covered, for up to 2 months. Shake well before serving chilled, or gently reheat before serving.
With a rubber spatula, scrape the marshmallow cream into a bowl.
Add 3 tablespoons of warm water to the jar, screw on the lid, and shake hard.
Add the liquid to the bowl and whisk until smooth. Thin the sauce with 1 to 2 tablespoons water if necessary. Use immediately or transfer to a glass jar and store at room temperature, tightly covered, for up to 3 months.
