In the saucepan, melt butter over low heat. Increase heat to medium & add flour, salt & cayenne pepper. Using a whisk, stir constantly, cooking until mixture is thick, smooth & bubbly, about 30 seconds. Add cream, in a slow stream, stirring constantly, cooking until smooth, thickened & drizzly, about 2 minutes. Turn off heat. Sprinkle in the Parmesan, stirring until mixture is smooth, adding milk/broth if necessary. Add the sherry, to taste. Add & toss into pasta mixture.