In a large pot of salted water, cook the pasta according to package directions until al dente.

While the pasta is cooking, prepare the sauce: In a frying pan over low heat, melt the butter. When the butter has melted, sprinkle on the flour and whisk it in, simmering for 2-3 minutes on medium low.

While whisking, slowly pour in the milk. Whisk until smooth, then increase the heat to medium and let the sauce to boil and thicken.

When the mixture is thick, remove from the heat and whisk in the sour cream until smooth.

When the pasta is done, use a slotted spoon to scoop it into the sauce. Toss until everything is fully coated and shiny. Thin the sauce with some of the pasta water if necessary. Mix in the chives thoroughly. Season with salt and pepper to taste. Enjoy while it's hot!