In a clean glass jar or bowl, combine 4 cups heavy cream with 4 tablespoons plain Greek yogurt. Stir gently until fully mixed. Cover loosely and let sit at room temperature for 12–24 hours, until slightly thickened with a mild tang.

Transfer the cultured cream to the refrigerator and chill for 1–2 hours. Cold cream separates more easily during churning.

Pour the chilled cream into a stand mixer, food processor, or large jar. Mix or shake on medium speed. The cream will first become whipped, then grainy, and finally separate into butter and liquid buttermilk. This takes 5–10 minutes.

Strain off the liquid buttermilk and reserve for baking if desired. The remaining solids are fresh butter.

Place the butter in a bowl of ice-cold water. Gently knead and press to release any remaining buttermilk. Drain and repeat with fresh cold water until the water runs clear.

Sprinkle the butter with flaky sea salt, to taste, and gently fold until evenly incorporated.

Shape into a mound or log, wrap tightly, and refrigerate for up to 1 week or freeze for longer storage.