Combine cream and yogurt in a 2-quart jar or container with a tight-fitting lid, cover, and shake to combine. Remove lid, cover with a clean kitchen towel, butter muslin or triple layer of cheesecloth, securing in place with a rubber band. Place jar in a warm place, preferably 75 degrees F, and let sit until thickened to the consistency of yogurt, 18 to 48 hours. (If temperature dips much below 75 degrees, culture may take up to 60 hours). Once thickened, remove the kitchen towel, cover jar with lid, and transfer to refrigerator until mixture registers 60 degrees F, about 2 hours.
Place 4 cups of ice water in the refrigerator. Line a fine-mesh strainer with a butter muslin or triple layer of cheesecloth and set it over a large bowl. Using a stand mixer fitted with the whisk attachment and covered with plastic wrap, whip cream on high speed until cream separates into buttermilk and small clumps of yellow butter, 2 to 5 minutes. Strain butter through prepared strainer for 1 minute. Gather edges of muslin and twist to squeeze butter until buttermilk no longer flows freely from pouch. Remove butter from muslin and transfer to a clean, large bowl; reserve buttermilk for another use.
Pour about 1/3 cup ice water over the butter. With butter resting in water, use rubber spatula to fold the butter against the side of the bowl, letting water wash over the butter to rinse off any remaining buttermilk. Discard milky liquid, and repeat washing process until water remains clear, about 6 washes. After the final wash, discard any water in the bowl and continue folding butter to squeeze out any remaining liquid; discard liquid. If you want to really get in there, squeeze the butter with your hands at the end. Sprinkle butter with salt, if using, and fold into butter. Divide the butter in half, transfer to parchment paper, and roll into 2 logs or desired shape. Butter can be refrigerated for up to 1 month for frozen for at least 4 months.
Shared with permission from ‘The America’s Test Kitchen DIY Cookbook’ by the editors at America’s Test Kitchen