Tear off 2 sheets of aluminum foil, about 2’ long each. Place one sheet on top of the other, and then place the sweet potatoes on top. Wrap up the sweet potatoes with the foil, crimping the seams to seal so that steam doesn’t escape. Place the foil packet on a rimmed baking sheet and place in the oven. Set the oven to 300 degrees F and roast for 2 hours, or until the potatoes are soft enough to mash.
Remove the sweet potatoes from the oven, open the packet, and let them sit until they’re cool enough to handle.
Peel the sweet potatoes and discard the skins. Place the potato flesh in a large bowl and mash with a fork. Add in the remaining casserole base ingredients and mix with a hand blender until well-combined and smooth.
Preheat the oven to 350 degrees F. Spray a square baking dish with nonstick spray and spoon the sweet potato mixture into the dish. Smooth it with the back of a spoon so that the sweet potato is all in an even layer. Set aside.
Place all of the pecan streusel ingredients except for the pecans in the bowl of a food processor and pulse until the mixture forms small crumbs. Add in the pecans and pulse again to combine evenly, making sure not to over process. Spoon the streusel over the casserole in an X shape, and then fill the open areas with small triangles, leaving at least an inch of open space between the edges, as shown in the pictures above. Bake for 30 minutes.
As the casserole bakes, prepare the marshmallow meringue. Beat the egg whites with the cream of tartar in a large bowl until stiff peaks form. Slowly add in the sugar while mixing until fully combined. Add in the marshmallow fluff and vanilla extract and beat again until sticky and still able to hold peaks.
When the casserole is ready, remove it from the oven and pipe or spoon marshmallow meringue into the empty areas. If you make a mistake, just scoop the marhsmallow off with a spoon and pipe again– you will have extra meringue so you can fix any little errors. Once you have the empty area filled in, add in an extra drop of marshmallow meringue in the bottom-center of each small streusel triangle as shown in the pictures above.
Return to the oven and bake until the meringue begins to brown, about 15-20 minutes. Serve warm.