Wonderful on a hot fudge sundae or chocolate cake.

Combine in a medium heavy saucepan:

Stirring, bring to a rolling boil. Remove from the heat and immediately add:

Stir gently until the marshmallows are melted. Stir in:

Cover and set aside. Proceed at once to make a water bath: Heat 1 inch of water in a large skillet over very low heat until it reaches 155° to 160°F and keep at this temperature. Combine in a heatproof bowl or the bowl of a stand mixer, stirring to dissolve the cream of tartar:

Whisk in thoroughly:

Set the bowl in the water bath and, whisking frequently, heat the mixture to 140°F. Whisk gently to maintain the mixture between 140° and 155°F for 5 minutes, moving the sauce off and on the heat as needed to maintain the temperature. Remove the bowl from the water bath and beat the egg white mixture on high speed (with the whisk if using a stand mixer) until the bottom of the bowl no longer feels warm, 4 to 8 minutes. Add the marshmallow mixture and beat for 30 seconds more. Use at once, or cool and cover tightly and refrigerate for up to 2 weeks.