Step 1

Preheat the oven to 400 degrees F.

Step 2

Put the potatoes in a large pot and add enough water to cover them by 2 inches. Stir in 1 teaspoon salt and bring to a boil over high heat. Simmer until you can smash them with the back of a spoon, 5 to 6 minutes. Drain the potatoes, add to a very large bowl and set aside.

Step 3

Meanwhile, melt the butter in a medium saucepan over medium heat. Add the flour and whisk to form a sandy mixture. Let the roux cook, untouched, until it's slightly toasted and smells a little nutty, 2 to 3 minutes.

Step 4

Mix the bouillon with 2 cups water in a large liquid measuring cup until dissolved. Add the milk, cayenne, 1 1/2 teaspoons salt and a generous amount of black pepper to the measuring cup. Whisking constantly, slowly pour the milk mixture into the pot with the roux and whisk until completely smooth. Cook, whisking frequently, until the flour taste has dissipated and the mixture is thick enough to coat the back of a spoon, 8 to 10 minutes.

Step 5

Turn the heat to low, add 2 cups of the Cheddar and cook, whisking constantly, until the cheese is melted and the sauce is very smooth, 2 to 3 minutes. Remove the pot from the heat and add the sour cream. Whisk until combined. Pour the sauce over the potatoes, add the scallion whites and fold the mixture until everything is evenly coated.

Step 6

Pour the mixture into a 9-by-13-inch baking dish and sprinkle with the remaining Cheddar. Scatter the cornflakes over the top and bake until the cheese mixture is bubbling on the sides, the cornflakes are slightly more golden and the potatoes are very soft, 45 to 50 minutes. Let sit for 10 minutes, then garnish with the scallion greens. Serve warm.