Culture cream: Combine yogurt and heavy cream in a large jar. Seal jar lightly and let the cream ferment for 24 to 48 hours.

Chill the cultured cream: Let the cream chill for at least 1 hour.

Churn butter: Pour the cream into a large mixing bowl or the bowl of a stand mixer. Whip the cream using a mixer with a whisk attachment until the mixture transforms into curds and the buttermilk begins separating.

Strain: Place a mesh strainer over a large measuring cup or mixing bowl. Pour the butter with the buttermilk into the strainer. Gently squeeze the butter through the strainer using a spatula. Try to squeeze as much of the buttermilk out as possible without squeezing it through the strainer.

Store buttermilk: Store the buttermilk in a jar and refrigerate.

Rinse butter: Place the butter into a bowl of ice water. Squeeze the butter to get rid of the buttermilk. Rinse the butter until the liquid runs clear. You must get the buttermilk out or your butter may go rancid.

Add salt to the butter: Place the butter into a mixing bowl and add salt. Using a wooden spoon, knead the salt into the butter until fully incorporated.

Store: Divide the butter into 3 portions, about 113 grams each. Place each piece of butter on a piece of parchment or wax paper. Form the butter into a cylinder and wrap it. Your butter will keep well in the refrigerator for 3 months or in the freezer indefinitely.