In the bowl of a completely clean stand mixer fitted with a whisk attachment, beat the egg whites with the cream of tartar on high speed until firm peaks form.
In a medium saucepan, combine the sugar, corn syrup, and water over medium-high heat. Attach a candy thermometer and cook the mixture, stirring occasionally, until the temperature reaches 240˚F, about 7-8 minutes. Immediately remove from heat.
Turn the mixer with the beaten egg whites on low speed. While it is running, slowly pour in the sugar syrup down the side into the egg whites. Don't do this too quickly or the egg whites will cook. Once all of the syrup has been mixed in, increase the speed to medium.
Continue to beat the mixture at medium speed until thickened and cooled, 10-12 minutes. During the last minute of mixing, add the vanilla.
Transfer the whipped marshmallow cream to a large pastry bag fitted with a round tip. Pipe the mixture in spirals or large puffs over the cooled marzipan base. Refrigerate the piped marshmallows for 1 hour to set.