1. Preheat Oven and Prepare Baking Dish:
* Preheat your oven to 375°F (190°C).
* Lightly grease a 9×13 inch baking dish with butter or cooking spray.

2. Prepare the Canned Yams:
* Open the cans of yams (sweet potatoes) and drain them very thoroughly in a colander. Gently press to remove excess syrup if needed, but be careful not to completely mash them at this stage. Discard the syrup.
* Transfer the drained yams to a large mixing bowl.

3. Create the Sweet Potato Base:
* Add the ½ cup of melted butter to the drained yams.
* Using a potato masher, mash the yams until they are mostly smooth. Some small lumps are perfectly fine and add to the rustic texture. Alternatively, you can use an electric hand mixer on low speed for a smoother consistency, but be careful not to overmix.
* Add the granulated or brown sugar (starting with ¼ cup and adjusting if desired), lightly beaten eggs, milk or heavy cream, vanilla extract, ground cinnamon, ground nutmeg, ground ginger (if using), and salt to the mashed sweet potatoes.
* Stir everything together with a large spoon or spatula until just combined. Avoid overmixing, as this can make the casserole dense. Taste a tiny bit (if comfortable with raw egg) and adjust sugar or spices if necessary.

4. Assemble the Casserole Base:
* Pour the sweet potato mixture into the prepared 9×13 inch baking dish.
* Spread it evenly with a spatula.

5. Prepare the Pecan Streusel Topping (If Using):
* In a separate medium bowl, combine the ½ cup all-purpose flour, ½ cup packed brown sugar, and pinch of salt (if using).
* Add the ¼ cup of cold, cubed butter.
* Using your fingertips, a pastry blender, or two forks, cut the butter into the dry ingredients until the mixture resembles coarse crumbs.
* Stir in the ¾ to 1 cup of chopped pecans.

6. Top and Bake:
* If using Pecan Streusel Topping: Sprinkle the pecan streusel mixture evenly over the sweet potato base in the baking dish.
* Place the baking dish in the preheated oven.
* Bake for 25-35 minutes, or until the sweet potato base is set (a knife inserted near the center should come out mostly clean or with moist crumbs) and the pecan topping is golden brown and crisp. The edges of the casserole should be slightly bubbly.
* If using Marshmallow Topping Instead: Bake the casserole without any topping for 20-25 minutes, until the base is mostly set. Then, remove it from the oven, sprinkle the mini marshmallows evenly over the top, and return to the oven for another 5-10 minutes, or until the marshmallows are puffed and golden brown. Watch carefully, as marshmallows can burn quickly. You can also broil them for 1-2 minutes at the very end for faster browning, but keep a very close eye on them.

7. Rest and Serve:
* Once baked, carefully remove the sweet potato casserole from the oven.
* Let it cool for at least 10-15 minutes before serving. This allows the casserole to set further and makes it easier to serve. The flavors also meld beautifully as it cools slightly.
* Serve warm.