The classic peanut butter and marshmallow sandwich—also known as the Fluffernutter—is inarguably all-American. And Le Petit Grain, a Parisian boulangerie headed by Edward Delling-Williams, created a delicious riff on that childhood favorite. The bakery combines all of the elements into elegant individual tartlets called tartes cacahuètes (literally, peanut tarts). A buttery, cookie-like pastry is filled with an airy peanut butter meringue that is topped with caramel-coated roasted peanuts. For ease, our version makes a single 9-inch tart. Pay attention to the timing in the recipe, which can be tricky. To make the meringue filling, the whipped egg whites and sugar syrup need to be ready at the same time. If your egg whites reach soft peaks before the syrup is ready, reduce the mixer speed to low while you wait for the syrup to finish; this prevents the whites from turning dry and stiff. You’ll need a candy or instant thermometer for gauging the doneness of the sugar syrup. The finished tart will keep at room temperature for up to 12 hours. If you're storing it longer than an hour or so, wait to add the flaky salt garnish until just before serving and cover the tart with plastic wrap or foil.
Don’t use natural peanut butter (the variety that requires stirring to mix in the oil on the surface); even the creamy variety of natural peanut butter has a slight grittiness that's detectable in the tart filling. Make sure the mixer bowl and whisk attachment for whipping the meringue are perfectly clean; even a trace of grease will prevent the egg whites from attaining the proper volume.