For the crust:
Preheat the oven to 350°F. Line an 8×8-inch baking pan with foil.
Place the graham crackers, sugar, and salt in the bowl of a food processor and pulse until finely ground. Add the melted butter and pulse until the mixture is moistened. Press the mixture into the bottom of the prepared baking pan. Bake for 10 minutes. Remove to a cooling rack and let cool completely. Keep oven at 350°F.
For the filling:
In a large bowl, use an electric mixer to beat all the sweet potato filling ingredients until well combined. Pour over the graham cracker crust and bake for 40 minutes, or until set but still slightly wiggly in the middle. Let cool to room temperature.
For the marshmallow topping:
In the bowl of an electric mixer fitted with the whisk attachment, beat the egg whites and cream of tartar until soft peaks form.
Meanwhile, in a small saucepan set over medium-high heat, combine the sugar, corn syrup, water, and salt. Bring the mixture to a boil and continue to cook until a candy thermometer registers exactly 240°F. Immediately remove from heat. Turn the mixture on low speed and carefully and gradually drizzle the syrup into the egg whites. Increase speed to medium-high and beat until the mixture is thick and glossy and looks like marshmallow fluff, about 7 minutes longer. Add in the vanilla and beat until combined.
Spread the marshmallow topping evenly over the sweet potato layer. Use a kitchen torch to gently torch the marshmallow layer until lightly golden. Alternatively, place the pan under the broiler and broil for 1 minute, or until lightly golden. Don’t step away – the broiler can go from toasted to burnt FAST!
Refrigerate the pan for at least 1 hour before cutting into squares and serving. The squares can be stored in an airtight container in the refrigerator for up to 5 days, though they are best served soon after the marshmallow is made.