Heat the 1 cup whole milk in a large saucepan over medium heat until small bubbles ring the edge of the pan. Stir in the 3/4 teaspoon sodium citrate until dissolved.
Meanwhile, melt the 2 tablespoons (1 oz) unsalted butter in another saucepan over medium heat. Sprinkle the 2 tablespoons all-purpose flour over the bubbling butter.
Stir constantly until a thick paste, or roux, forms. It may become quite thick and almost paste-like. This is okay. And if it doesn’t become crazy thick, that’s okay, too. About 1 to 2 minutes.
Constantly whisking, slowly pour the warm milk into the flour and butter mixture in a steady stream.
Continue cooking and whisking until the mixture thickens, 3 to 5 minutes. It should be thick but still spreadable and pourable.
Stir in the 1 cup grated cheddar cheese and whisk until completely melted.
Season with the 3/4 teaspoon ground cayenne or hot sauce and sea salt and freshly ground black pepper to taste. Whisk until the sauce is smooth.