Add the cream with the salt (optional) in a large bowl and start beating on low speed using the electric mixer or hand balloon whisk. Gradually increase the mixer speed and continue beating for approximately 7-10 minutes until the mixture thickens.
After about 5 minutes, a whipped cream with stiff peaks will form but continue to whip until the mixture turns a slightly yellow color. After several more minutes, the fat solids in the cream will separate from the liquids and then the butter will begin to form, leaving the buttermilk at the bottom of the bowl.
Strain the solid part (which is the butter) from the buttermilk with a fine mesh strainer or cheesecloth. You have to be careful and let all the liquid you can out, as the more liquid stays in the butter, the faster it will spoil. The buttermilk can be reserved and used in other recipes.
Place the butter in a bowl of ice cold water and rinse it a little with the same water. Discard the water and put more ice cold water back into the bowl with the butter to repeat the rinsing process, which should be done a total of three times.
Form the butter into a ball or give it any shape you prefer. At this point the butter is ready and can be seasoned with various flavors or used for other preparations.