I found this recipe years ago in a book that is sadly now out of print. When I get all my books unpacked again, I'll try to remember to update this with the correct source. Edited to add: The source is Cheaper & Better Homemade Alternatives to Storebought Goods by Nancy Birnes.
1 lb softened butter or margarine--- I always use butter.
1 cup vegetable oil--- I use extra light olive oil.
1 cup buttermilk-- I use powdered buttermilk and add the water in with the oil.
Unwrap the softened sticks of butter or margarine and place in a mixing bowl or your food processor. If using your mixer, begin beating the butter or margarine till it is fluffy and *slowly* add the buttermilk and oil. Continue beating until it is nice and smooth.
If using your food processor, place all of your ingredients in the work bowl.
Then add the lid and process until smooth. Scrape it out into the covered container of your choice and store in the refrigerator.
This will keep for about a month in the refrigerator. Use as you would any commercial butter or margarine spread that comes in a tub. I don't recommend baking with it as you will likely be disappointed with the results.