Easy blender method for making buttermilk out of heavy whipping cream. One pint of cream produces a half pound of butter and one cup of plain or cultured buttermilk.
Pour cream into the blender or food processor. Secure the lid firmly.
Turn on low and let the cream churn for 5-10 minutes. If your cream is very fresh and cold, it will take longer to whip into butter.
Once you can see the separation of butter from buttermilk is complete, turn off the blender and carefully pour the buttermilk into a bowl. If you use a fine mesh cheesecloth, you will separate out all the small bits of butter as well.
If you want cultured buttermilk and you used raw cream, leave the strained buttermilk on the counter in a sealed mason jar for 1-2 days. It will naturally ferment into cultured buttermilk.
If you used pasteurized cream, add buttermilk starter and use as directed.
You will end up with about 1 cup of butter and 1 cup of buttermilk.
Do not use UHT ultrapasteurized cream even if organic. It is denatured for excessively high heat processing and sometimes even contains unnatural thickeners.
This article on UHT milk explains more.