Mix the butter and salt with a fork until the mixture is well blended.
Reform the butter, either a whole slab or smaller slabs, and return to the fridge.
Notes
The amount of salt you use depends on what you want. I recommend preparing a whole packet (200 grams), as it's easier to measure out, especially for the semi-salted variety.Whether it's fine salt, coarse salt... the amount of salt is always the same, and only the weight counts.It's important that the butter is just softened and not melted. Once it's melted, its structure has changed and even if it's hardened again in the fridge, it won't be the same. One exception: clarified butter.