Beat the butter with a hand-held or stand mixer on a low speed, until it becomes soft and creamy. Add the bottarga and chives and set aside.
Bring a large pan of salted water to the boil and add the spaghetti. Cook them for about 5 minutes, drain the water (save some if you don’t have fish stock) finish cooking them in a pan with fish stock (or some of their cooking water). It’s important to add the liquid little by little as if you were cooking a risotto. When done add the butter cream and stir.