Pour the contents of one carton of whipping cream into a deep bowl. Beat with mixer on high. The cream will gradually increase in volume, then decrease. After approximately 10 to 12 minutes, it will begin to separate into buttermilk (a thin milky fluid) and butter. Pour off the buttermilk and the remaining substance is butter. Add 1/8 teaspoon of salt and mix until well blended. (Use more or less salt according to preference.) If you do not have a butter mold, press into a small butter cup or any plastic container.