I recently bought a case of organic whipping cream through a food co-op. At the time, I had planned to use it for our cupcakes at the farmers market. But, things didn't go as planned and I was only able to make it there once. Boo.
I could have freezed it to use for cooking later, but I wanted to get creative. A friend had told me about making butter with whipping cream by shaking a jar with a marble in in. As (un)lucky as I am, I really didn't want to combine shaking and glass so I broke out our mixer.
I chilled the bowl and beater in the fridge for a bit and the dumped in two pints of whipping cream. I turned the mixer on high for a few minutes. At first, it just turns into whipped cream but it just keeps getting thicker and thicker.
All of a sudden, the milk and solids start separating. I learned an important lesson at this point:
Stay next to your mixer during this process. If you are across the room when everything starts separating, the milk will splatter all over your counter, cupboards and walls!
Once it separates, drain away the milk (I kept the milk for cooking - it's pretty much just skim milk) and you need to 'wash' the butter. To do this, rinse the butter under cool water until the water is clear. You'll have to squeeze the butter while rinsing it to make sure all the milk washes off. If you don't do this, the butter will spoil faster. Store in the fridge in an air tight container.
I'm not entirely sure how long it will last in the fridge, but we used ours for two weeks without any issues.