Note on homemade spreadable butter. Do not leave butter at room temperature for too long or it will separate. If this happens, stir the oil back into the butter before refrigerating again.
To make your own additive free soft butter mix together 50/50 room temperature butter and olive oil or other vegetable oil. For a firmer mix go 2/3rds butter to 1/3rd olive oil. Mix in food processor, a hand blender, or use kitchen utensil and elbow grease.
I only buy soft butter in winter. Otherwise I buy blocks of butter and slice off what I need for a few days and store at room temperature. 500g of butter costs $2.50-$3.00 compared to 500g soft butter on special at $4.00.