Add cream directly into the blender cup. Seal the lid on tight.
Blend the cream on high. Keep blending past the whipped cream stage, and eventually the watery buttermilk will separate and you will start to see butter chunks forming and sticking to the sides of the blender. When you see bright yellow globs, that's a good sign that you are done!
Once you start to see yellow globs of butter forming, turn off the blender.
Pour off the excess buttermilk. Save it for future recipes! Scoop the butter out of the blender, and put it in a glass bowl.
Using your hands, rinse and squeeze the butter under COLD water in the sink. Squeeze out all of the buttermilk.
Place the butter back in the bowl, and add salt if desired. Taste it and continue adding until you like the flavor.
Store and Enjoy! If you like softer butter, you can keep it in an airtight container on the counter for up to a week. If you want your butter to last longer, wrap it in parchment paper and keep it in the fridge for 2-3 weeks.