Begin by preheating your oven to 375°F and lightly greasing a 9 x 13-inch baking dish.
Place the cubed sweet potatoes into a large pot of boiling water. Allow them to simmer for about 15 minutes, or until they become tender when pierced with a fork. Drain well.
Transfer the drained sweet potatoes to a large mixing bowl, or you can mash them directly in the pot they were cooked in. Add the softened butter, brown sugar, vanilla extract, cinnamon, and a pinch of salt and pepper. Mash the mixture until smooth and well combined.
Gently fold in half of the chopped pecans, then spread the sweet potato mixture evenly into the prepared baking dish.
Top the casserole with the miniature marshmallows, followed by the remaining chopped pecans.
Bake in the preheated oven for approximately 25 minutes, or until the marshmallows are golden and the casserole is thoroughly heated.
Serve warm, and enjoy the delightful combination of flavors and textures in this comforting dish.