Peel the sweet potatoes and cut them into 1-inch cubes for even cooking. Place them in a large pot and cover completely with cold water. Bring to a boil over high heat, then reduce to a simmer and cook for 10-15 minutes until the potatoes are completely fork-tender. Drain thoroughly in a colander, pressing gently to remove excess moisture—this is important because watery potatoes will make your casserole soggy. While the potatoes cook, you can prepare your other ingredients and preheat the oven.

Preheat your oven to 375°F and position the rack to the middle position. Spray a 9×13-inch casserole dish (or similar size) with cooking spray to prevent sticking. This step should be done while your potatoes are cooking so everything is ready when you need it.

While the potatoes are still warm, transfer them to a large mixing bowl. Add the softened butter, brown sugar, room-temperature milk, eggs, vanilla extract, and salt. Using an electric mixer on medium speed, blend everything together until you achieve a smooth, creamy consistency—this usually takes 2-3 minutes. I find that room-temperature milk incorporates much more smoothly than cold milk, creating a velvety texture. Once smooth, gently fold in the 1/4 cup finely chopped pecans by hand to maintain the creamy texture and distribute the nuts evenly throughout.

Spread the potato mixture from Step 3 evenly into your prepared casserole dish, using a spatula to create a level surface. Scatter the miniature marshmallows evenly across the top—I like to leave small gaps between them so some peek through while others meld together during baking. Finish by sprinkling the optional 1/4 cup chopped pecans over the marshmallows for added crunch and richness. These pecans will toast as the casserole bakes, creating a delicious nutty flavor.

Place the assembled casserole in your preheated 375°F oven and bake for 25-30 minutes until the marshmallows are golden brown on top and the edges are bubbling slightly. The exact time depends on your oven, so start checking around the 25-minute mark. The casserole is done when the marshmallows have puffed up and turned a light golden color, and the sweet potato mixture is hot throughout.

Remove the casserole from the oven and let it rest for 5 minutes before serving. This allows the structure to set slightly, making it easier to scoop and serve while keeping the marshmallows warm and gooey. Serve directly from the dish while warm for the best flavor and texture.