Cook the pasta as per package instructions. Drain and keep aside, but reserve some of the pasta water for loosening the sauce later if required.
In a large pan, melt the butter on low heat and add the garlic. Sauté for a few seconds. Don't let the garlic brown up.
Add the flour little by little, stirring as you go along so it doesn’t clump up.
Add the milk and let the sauce thicken. Stir it continuously as the sauce will thicken in a few seconds. If the sauce becomes too thick, add some of the reserved pasta water. If using the black pepper and red pepper flakes, add them in now. Take the pan off the heat and let the sauce cool down.
Stir in the sour cream and mix it into the sauce, using a spoon.
Add the cooked pasta, olive oil, and chives and mix to coat with the sauce. Top with more chives and serve warm.
Notes
Keep the heat low and slow at all times while making the sauce. Your aim is to make sure the garlic and flour gets cooked nicely without any browning. This makes for a golden sauce without anything tasting raw.
Reserve some pasta water while draining the pasta. If the sauce gets too thick, this pasta water comes in handy in loosening it up.
The garlic does burn quite easily, so keep an eye on it while sautéing it.
Don’t dump the flour all at once into the butter. To make a smooth roux, use a sieve to add the flour in increments and keep stirring it while it is cooking to prevent lumps.