I buy the milk from a local farmer, let it sit in the refrigerator overnight until the cream completely rises to the top and then skim off the cream (this is what you whip into butter)...
Here is cream from 1 gallon milk in January 2011. In the spring & fall, the cream will be much more yellow (and, therefore, so will the butter) due to the rapidly growing grass. Incedently, butter in the spring and fall contain a much higher amount of vitamins and as well as other good stuff.
The following pictures were taken last fall. As you can see, the cream is a brighter yellow.
After pouring some cream in the food processor, I let it whip anywhere from 6-15 minutes (depending on the temperature of the cream).
Now the cream has separated into 2 parts: butter & buttermilk. I pour the mixture into a colander, placed over a bowl that catches the buttermilk.
I squeeze the globs of butter a little and then place in a bowl of very cold water.
I "wash" the butter & squeeze out any leftover buttermilk several times until the water I squeeze out is relatively clear. I'm not too much of a stickler on this.