Preheat your oven to 375°F and lightly grease a 9x13-inch baking dish. The prepared baking pan ensures effortless serving and prevents the sweet potato mixture from sticking.

Submerge the cubed sweet potatoes in a large pot of boiling water, simmer for 15 minutes, and drain thoroughly in a colander. The potatoes are ready when a fork slides into the center of the largest cube with zero resistance.

Mash the drained hot sweet potatoes with the brown sugar, softened butter, ground cinnamon, vanilla extract, salt, and black pepper until completely smooth. The warm steam will release a fragrant sweet spice aroma as the butter melts cleanly into the velvety mash.

Fold half of the chopped pecans into the potato mixture and spread the filling evenly across the bottom of the prepared dish. An even layer ensures consistent heat transfer and prevents hot spots during baking.

Scatter the miniature marshmallows and the remaining chopped pecans uniformly over the entire surface. The marshmallow layer should form a complete blanket without bare patches along the edges.

Bake in the preheated oven for 25 minutes until the casserole is heated through and the topping turns toasted golden brown. The marshmallows will puff gently and display a speckled amber crust across the surface.