Use unsalted butter or cooking spray and grease the 11x7-inch baking dish; you can also use non-stick cooking spray.
Drain the sweet potatoes, add the drained sweet potatoes into a large bowl, and fold in the sugar, butter, salt, ground cinnamon, and vanilla extract. Use a potato masher and mash the potatoes to the desired consistency. You can also use an electric hand mixer, electric mixer, or food processor. Fold in 1/3 cup of diced pecans.
Pour the mashed sweet potato mixture into the prepared baking dish, add about 1/4 cup of diced pecans on top of the casserole, and sprinkle the mini marshmallows.
Place the casserole dish into a 375 degrees F, preheated oven, and cook for 25-30 minutes, or until the casserole is warmed and the marshmallows are golden brown.
Serve with a drizzle of maple syrup or melted butter over the top, with a dash of dark brown sugar or a little bit of ground cinnamon!