How to soften homemade butter
I make butter spreadable-from-the-fridge by adding grapeseed oil (recipe here -- 1:1 ratio for very soft "butter") and blending in a food processor for a smooth consistency.

This is done after the butter is churned, or after leaving store-bought sticks to come to room temperature. Pretty much any oil can work for this, but a neutral-flavored oil will keep the mixture tasting like butter.

I don't recommend using this for anything besides buttering bread; changing the consistency in this way can affect baked goods.