To make a flavoursome healthy spreadable butter straight from the fridge, use extra virgin olive oil.
125ml (1/2 cup) extra virgin olive oil
Allow the butter to soften and add butter and olive oil to a bowl. Blend with a stick blender until smooth and fully blended. Add to a suitable container to store in the fridge.
You will find the spread has the added taste of the olive oil so ensure you use an extra virgin olive oil with a complementary flavour to the butter.
If you find the above ratios do not make a spreadable butter to you liking, add more or less olive oil; more to make softer, less to make harder.