In my post How To Turn One Stick Of Butter Into Two, I explained how to make extended butter. This extended butter should not be confused with soft butter spread. Extended butter is made by thoroughly combining equal parts of butter and water and will be the same texture and consistency of the butter you started with. The extended butter will be as firm as regular butter and will not be any easier to spread straight out of the refrigerator.
However, if you want to make an easily spreadable extended butter, just substitute oil for some or all of the water. I usually choose a mild-flavored olive oil, but any healthy oil will do. Using a hand mixer, thoroughly mix the oil and butter until neither one is separately identifiable. You will end up with a soft spreadable extended butter that will stay soft even when it is refrigerated.
Butter extended with oil will, of course, contain more fat and calories than the original butter and water method, but it is a good substitute for the commercial soft butter spreads. It’s also great for those of us with soy allergies, because it contains only soy-free butter and oil.