Adjust oven rack to middle position and heat oven to 375°F. Spray a 13x9-inch casserole dish with nonstick cooking spray.
In a small bowl, combine Parmesan, bread crumbs, melted butter and ¼ teaspoon salt; set aside.
Combine potatoes, 3 cups cream, baking soda, 2 teaspoons salt and 1 teaspoon black pepper in a large saucepan or Dutch oven.
Bring to a boil over medium heat and reduce heat to low.
Cook at a bare simmer, stirring often, until potatoes have softened to fork tender but not crumbling, about 25-30 minutes.
Remove from the heat and add remaining ½ cup cream and 6 tablespoons butter. Stir gently until butter has melted, about 1 minute.
Pour the mixture into the prepared casserole dish and sprinkle evenly with the Parmesan/Panko mixture.
Bake, uncovered, until golden brown and bubbling around edges, 20-30 minutes. Allow to cool 10-15 minutes before serving.
Enjoy!