Pour cream into a clean quart-sized jar, and add 1 tablespoon of the cultured buttermilk, kefir, or yogurt. Mix well with a clean utensil, and cover jar loosely with a lid or a clean cloth.
If culturing the cream with buttermilk or kefir, place the jar in a warm place in your kitchen. If you are culturing the cream with yogurt, you will need to place your jar in a warmer place. For example, you can place your jar in your oven, keep your oven turned off, and have your jar warmed by the oven's pilot light or electric light on. (You want to maintain a temperature of about 110°F when culturing cream with yogurt.)
Allow the cream to culture anywhere from 8 to 48 hours. Once you achieve the desired level of taste, transfer the cultured cream to the device you will be using to whip it, such as a stand mixer. Whip the cream until butter forms and a white liquid is left in the bottom of the mixer bowl. This white liquid is real buttermilk.
Line a colander or mesh strainer with cheesecloth or a flour sack towel and place the lined strainer over a large bowl. Strain the butter and buttermilk through the lined strainer. Pull the cloth together and squeeze out as much buttermilk as possible from the butter. (See the video for the process.)
Next, transfer the butter to a clean bowl and cover it with cold chlorine-free water. Press the butter with a spatula to release any bits of remaining buttermilk. The water will become cloudy. Pour off the water and repeat this process until the water remains clear and no more buttermilk can be pressed out of the butter. Drain off all water. Press the butter to release any remaining water and pour it from the bowl.
If you wish to salt your butter, add in some sea salt at this point and work it into the butter.
Transfer the butter to a surface covered with wax paper, parchment paper, or plastic wrap. Form the butter into a rectangular shape. Wrap the butter well, or put it in a butter dish and refrigerate. The butter should remain fresh for approximately two weeks.