Cultured Butter

In a large bowl, whisk the heavy cream with the buttermilk or crème fraîche. Cover with plastic wrap and let stand at room temperature until thickened, at least 12 hours and up to 48 hours.

Scrape the cultured cream into the bowl of a stand mixer, cover with plastic wrap and refrigerate until well chilled, about 45 minutes.

Wrap the rim of the bowl well with plastic to catch any splatter; alternately, use the pouring-shield attachment. Beat the cultured cream with the whisk at high speed until the butter solids start to form a ball, about 4 minutes. Drain the butter solids in a fine sieve set over a bowl and reserve the buttermilk for another use.

Transfer the butter to a large bowl and, using gloved hands or a large fork, knead to expel any excess buttermilk, adding it to the reserved buttermilk as it accumulates. Pour 1/4 cup of ice water over the butter, knead and drain. Repeat adding the ice water, kneading and draining 3 more times, until the water is nearly clear. Continue kneading the butter until it no longer expels any water.

Form the butter into a cylinder or block, wrap it in cheesecloth and gently squeeze to remove any remaining moisture. Discard the cheesecloth and wrap the butter tightly in plastic, followed by parchment paper. Refrigerate for up to 1 week.