Bake one cookie sheet at a time in the center of the oven until cookies are lightly browned on edges (approx. 10-12 mins).
Remove from oven and let cool for 5mins before transferring to a cooling rack to cool completely.
Place chocolate chips and cream into a microwave safe bowl. Stir to combine. Microwave for 30 seconds, stir. Microwave in 10 second intervals, stirring in between, until ganache is smooth and silky. Set aside to cool completely and thicken before using.*
Place water, sugar, and corn syrup in a medium saucepan. Stir to combine.
Insert a candy thermometer into the pot and heat over medium-high. Do not stir from this point on as crystals will form.
Ensure mixer bowl and whisk are completely grease free.**
Place egg whites and cream of tartar in the bowl of a stand mixer.
When the sugar syrup reaches about 225°F, start whipping the egg whites to soft peaks. Approx 3-4mins.
When the whites are ready, the sugar syrup should be at 240°F. Remove from the heat, turn mixer to medium and very slowly and carefully pour the sugar syrup into the whites in a thin, steady stream.***
Once all of the syrup is in, set mixer to medium/high and continue whipping. The whites will deflate at first, but they will thicken and fluff up.
Continue to whip for 7-8 minutes, or until the mixture is thick and glossy. Add in vanilla and whip until the fluff has cooled.
Spread chocolate ganache on one cookie and pipe marshmallow fluff on another. I used a piping bag fitted with an Ateco 808 tip, but you could also just spread this on.
Carefully toast the marshmallow using a kitchen torch and sandwich the two cookies together.
* I place plastic wrap directly on top of the ganache and let mine sit overnight to thicken. You can speed up this process in the fridge. Once cooled, if ganache is too thick to spread, you can microwave it for 5-10 seconds to thin it out. ** If there is even a speck of grease (or egg yolk) the meringue will not whip. I wipe everything down with lemon juice before adding the egg whites. *** The sweet spot for this is getting the stream right between the whisk and the sides of the bowl, so the syrup doesn't get splashed by the whisk.You may have some marshmallow fluff left over which you can store in an airtight container in the freezer and use it at a later date for S'mores Macarons, S'mores Cookie Cups, or good old Rice Krispie Treats.