This will allow you to keep butter without refrigeration for up to 6 months.
To make brine: Boil 2 cups of water and add 1/4 C of non-iodized salt (pickling or kosher salt works fine). Allow to cool.
Sterilize several wide-mouth jars and lids in boiling water. Set on a towel on the counter and allow to cool slightly. Fill each jar with softened butter, making sure there are no air bubbles. Allow an inch of headspace at the top, and fill with the cooled brine.
When you use the butter, dip out each serving with a clean utensil. Replace the volume used with fresh water to keep the jar full.
Note: When I started doing this, I used only one pound of butter at a time. That means I don’t need nearly as much brine. So I cut the quantities down to 1/2 C of water and 1 T of salt boiled together.