Sweet Potatoes: Preheat oven to 350 degrees F (176 C). Bring a large pot of water to a boil. Chop the sweet potatoes then throw them in. Cook for 15 to 20 minutes, or until fork tender, then drain. Mash until smooth then add butter, milk, vanilla, cinnamon, and salt. Stir until combined and set aside until cooled (room temperature), then stir in the egg yolks (reserve the whites).
Bake: Pour sweet potato mix into a casserole dish. In a small bowl combine butter, pecans, flour, and brown sugar, then sprinkle on top. Bake for 25 minutes, or until pecan crumble is crisp.
Meringue: Meanwhile, add egg whites to a clean bowl and begin beating at medium speed. When they become frothy, add the tartar and begin adding sugar. Gradually add the sugar until fully incorporated, beating until firm, glossy peaks form. When pecan crumble is crispy, spread meringue on top. Return to the top rack of the oven for 3 minutes, until golden brown (watch closely to prevent burning!)
*For a sweeter casserole, double the pecan crumble ingredients.