A ribbon of fluffy homemade marshmallow spread swirls through rich vanilla custard ice cream, the perfect filling for a s'more ice cream sandwich. (See "tips," below for details on how to make toasted s'more ice cream sandwiches.)
To make the ice cream base: Pour the milk and cream into a saucepan set over medium-high heat and bring the mixture to a boil. Add the vanilla, remove from the heat, and let sit in the saucepan for 30 minutes.
In a separate saucepan, beat together the egg yolks, sugar, and xanthan gum, until light and airy. Stir the warm milk and cream into the egg yolk mixture.
Place the pan over low heat and cook the mixture, stirring, until it thickens slightly; it will register between 170°F and 180°F when measured with a digital thermometer. Remove the pan from the heat and let the custard cool to room temperature, stirring occasionally to prevent a skin from forming. Cover and refrigerate for at least 2 hours, or overnight.
To make the marshmallow swirl: Place the egg whites and cream of tartar in the bowl of an electric mixer fitted with a whisk attachment and set aside.
In a medium saucepan, combine the water, sugar, and corn syrup or honey. Stir to combine. Insert a candy thermometer and heat the mixture over medium-high heat. Don't stir the mixture as it cooks or it will become grainy.
When the sugar syrup begins to form large bubbles and the thermometer reads about 225°F, begin to beat the egg whites to soft peaks (this should take about 3 to 4 minutes). By the time the egg whites are done, the sugar syrup should be at 240°F. Remove the syrup from the heat and, with the mixer running at medium-low speed, carefully pour it down the side of the bowl into the whites in a thin, steady stream.
The whites will deflate a little at first, but continue to beat and the mixture will begin to expand. Whip for 7 to 8 minutes, until the spread is very thick and glossy. Add the vanilla and whip for 1 more minute. Pour the spread into an airtight container and store at room temperature.
To finish the ice cream: Transfer the chilled custard to an ice cream maker and freeze according to the manufacturer's instructions. Once the ice cream begins to thicken but is still soft, fold 1 1/2 cups of the marshmallow spread into the ice cream, leaving discernible streaks throughout.
Place the finished ice cream in a container with a tight-fitting lid and freeze for up to 2 months.
To make toasted marshmallow swirl ice cream: Prepare the ice cream base and marshmallow swirl as directed. Before combining the two, use a brûlée torch (blow torch) to toast the marshmallow until it's golden brown in areas. Swirl it into the ice cream and chill as described (step 9).
To make s'mores ice cream sandwiches: Prepare the ice cream as directed, toasting the marshmallow swirl (see tip above for details). Prepare a batch of graham crackers (homemade or store-bought). Scoop about 1/4 cup of the marshmallow swirl ice cream onto one half of a graham cracker; place the other half on top and gently press until the ice cream reaches the edges of the crackers. Freeze the assembled sandwiches while you prepare the fudge coating. Melt 1 2/3 cup (284g) semisweet chocolate (chips, wafers, or chopped from a bar) with 1 tablespoon of coconut oil. Stir until the mixture is glossy and smooth, then allow to cool to lukewarm before dipping the ice cream sandwiches. Serve immediately or freeze, well-rapped, for up to a month.
