Ingredients
double cream

Method
1. Take the cream out of the fridge and let it warm to room temperature for about half an hour.
2. Pour enough cream into a jam jar (choose the largest jar you can comfortably hold) to come a third of the way up the sides. You need to leave plenty of airspace so the cream can really move around.
3. Screw on the lid tightly. Now shake the jar up and down and all around so that the cream bounces against the lid. It is important not to stop shaking until the butter starts to form.
4. First you will feel the cream slop around in the jar, then you will notice that it stops slopping and goes silent. At this stage you just have whipped cream. Keep shaking. Pretend you're playing the maracas! It may take ten minutes or it may take half an hour.
5. All of a sudden the sound and the sensation will change. You'll have a big lump sloshing around in a thin, watery liquid. The lump is your butter and the liquid is buttermilk. Carefully open the lid and take a look inside.
6. Now you have to wash your butter under the cold tap. Drain the buttermilk off into a mug and fill the jam jar with cold water. Swirl the lump of butter around in the water and pour the water carefully away. Do this again and again until the water is clear.
7. Put the butter lump on the board and press down on it with the back of a wooden spoon (or use your hands) to force out any buttermilk still inside. This is important, as any buttermilk left inside it will make it go sour.
8. You can now mould your butter into a shape, wrap it in greaseproof paper and keep it in the fridge, or eat it straightaway.