Heat the oven to 350°F and grease a 9-by-13-inch dish with 1 teaspoon of the butter. Cover potato cubes with cold water by 1 inch, add cooking salt, bring to a simmer and cook for 6 to 8 minutes until just tender; drain and let steam escape for 3 minutes.
Melt 2 tablespoons butter in a saucepan over medium heat and cook celery and onion for 6 minutes until soft. Stir in flour for 1 minute, then gradually whisk in milk and broth and simmer for 3 minutes until lightly thickened.
Remove the sauce from heat and stir in sour cream, cheddar, sauce salt and pepper. Fold through the drained potato cubes and spread evenly in the dish.
Melt the remaining butter and toss with the crushed cornflakes until evenly coated. Scatter over the potatoes in a loose layer.
Bake for 30 to 35 minutes until the center bubbles and the cornflakes are golden. Cover loosely if the flakes darken early, then rest for 10 minutes before scooping.