Half fill a screw top jar with double cream and secure the lid.
Shake the cream for around 10 minutes or so. First, the cream will turn to a stiff whipping cream consistency, then the buttermilk will separate from the butterfat. You'll know when this happens as you'll suddenly hear the buttermilk sloshing about.
Tip the contents of the jar into a fine mesh strainer to strain our the buttermilk. The buttermilk is delicious to drink on its own, or you can use it in recipes. Use a paper towel to dab away any remaining buttermilk (or your butter may spoil quicker).
Eat and enjoy or place your butter in an airtight container and refrigerate as normal.
You can also use a blender for this recipe. I love using a jar, as it adds to the experience, especially if you're making the butter with kids, but if you want to make a larger amount of butter, or don't want to give your arms a workout, a blender works just as well.