Step 4
In a large saucepan or Dutch oven, heat the butter over medium-high heat until melted. Add the flour and cook, constantly whisking, until it turns light and toasty brown color, 3 to 6 minutes. Gradually add the milk while whisking, then increase the heat to high and bring it to a boil. Reduce the heat to medium and cook, whisking, until it thickens, 4 to 5 minutes. Add the cheeses, a few handfuls at a time, and whisk the sauce until smooth. Whisk in the salt, black pepper, and cayenne. Stir in the cooked pasta, and season to taste with more salt and pepper.