Preheat oven to 425ºF.
Wrap each potato in aluminum foil and pierce several times with a fork. Bake the potatoes right on the oven rack until fork tender, about 1 hour. Remove the foil and let the potatoes cool slightly. Cover with plastic and put them in the refrigerator to cool completely.
When potatoes are completely cooled preheat oven to 375 degrees F. Butter a 9 x 12 casserole or baking pan.
Peel and cut the potatoes into 1/2-inch cubes.
Whisk sour cream, cream cheese, cornstarch, salt, pepper and paprika together in a large bowl until combined - it may look a little curdled - that's ok. Whisk in the chicken broth.
Add the potatoes, scallions and 2 cups of the cheddar. Stir carefully until well combined. Pour into the casserole dish and sprinkle the remaining 1/2 cup of cheese over top.
Bake 30 - 35 minutes until bubbly and lightly golden on top. Let set 5 - 10 minutes before serving.