Preheat the oven to 350F. Lightly grease a 9-inch pie plate. In the bowl of a food processor, add all the ingredients for the crust except the melted butter. Pulse 5-6 times until the mixture forms crumbs. Add the butter and pulse 2-3 times until incorporated.
Press the crust evenly into the prepared pie pan. Bake for 20 minutes. Set aside.
Make the Custard:
Meanwhile, cook the sweet potatoes for the filling.
In the INSTANT POT (preferred method): Place a rack in the instant pot and add 1 cup of water. Place the washed sweet potatoes on top of the rack. Cook on MANUAL (high pressure) for 15 minutes. Allow the sweet potatoes to natural release, until the valve drops on its own.
On the STOVE TOP: Place the sweet potatoes (whole and unpeeled) in a large pot and covering with water. Cover with a lid and bring to a boil. Boil for 10 minutes. then puncture each sweet potatoes 3 times. Boil for another 20-30 minutes until cooked through.
Peel the sweet potatoes and place them in a bowl. Allow them to cool slightly then add all the ingredients except the eggs. Using a hand mixer, whip the sweet potatoes until smooth. Add the eggs and continue to whip until very smooth.
Pour the sweet potato custard into the pie crust and smooth the top. Bake for 40 minutes, until set. You may need to cover the top/edges of the crust with foil to prevent over-browning.
Let the pie cool at room temperature for 1 hour. Then transfer to the fridge and let cool for at least 4 hours before topping with the meringue.
Make the Meringue:
Position an oven rack in top third of oven and preheat the oven to 400°F. Scrape the marshmallow creme into a large bowl. Using a hand mixer, beat the egg whites, salt and vanilla in another large bowl until foamy. Add sugar, 1 tablespoon at a time, and beat until stiff and glossy peaks form.
Add ½ cup of the beaten egg whites to marshmallow creme and stir with a rubber spatula until incorporated to lighten the marshmallow creme. It will be very sticky at first but will get easier to fold in as you add the egg whites.
Gently fold in remaining whites in two additions, until just incorporated being careful not to stir too much. Spread the meringue over top the cold pie, mounding slightly in center and swirling with the meringue to create peaks.
10. Bake the pie for 5-7 minutes until the meringue is set and starts to turn golden brown. Let cool at room temperature for 1 hour before serving. Once the pie has cooled for an hour, you can store it in the refrigerator for up to 6 hours before serving.
MAKE IT DAIRY-FREE: Substitute the butter for vegan butter and the heavy cream for coconut milk. My favorite way to steam the sweet potatoes is in the instant pot.It’s so easy to do and the sweet potatoes come out perfect every time. You’ll just want to be sure to make the meringue no more than 6 hours before you want to serve the pie.The crust and filling can be made the day before, but the meringue is best eaten the day it is made.