We prefer the flavor of butter made with pasteurized cream as opposed to ultrapasteurized cream. The ideal temperature range for churning butter is 55 to 60 degrees; colder and the fat is too firm and will stick to the sides of the food processor bowl; warmer and the fat is liquid instead of solid, leading to greasy butter. In step 2, chill the cream in the refrigerator or over an ice bath. For the most complex tangy flavor, we recommend aging the cream for a week. At that point the cream may smell quite pungent, but most of what you smell resides in the liquid that gets separated out, leaving the butter surprisingly mellow. This recipe requires cheesecloth.
1. Combine cream and buttermilk in clean lidded container. Cover container and let sit at room temperature until mixture smells tangy and buttery and thickens to sour cream–like consistency, at least 24 hours or up to 1 week.
2. Chill cream mixture to 55 to 60 degrees.
3. Process cream mixture in food processor until mixture turns from grainy whipped cream to lumps of butter splashing in liquid, 1 to 3 minutes. Stop processor immediately.
4. Fill medium bowl halfway with ice and water. Line fine-mesh strainer with triple layer of cheesecloth, leaving few inches of cloth hanging over sides of strainer, and set over a large bowl. Drain butter mixture in prepared strainer (buttermilk will collect in bowl). Lift cheesecloth by edges and twist and squeeze tightly over strainer to press out more buttermilk (stop when butter starts to squeeze through cheesecloth). Transfer cheesecloth-wrapped butter to ice bath until firm around exterior, about 2 minutes. Transfer buttermilk to airtight container.
5. Remove butter from cheesecloth and transfer to now-empty bowl. Stir and press with wooden spoon (metal utensil will conduct heat from your hands and make butter soft) to force out additional buttermilk from butter, 1 to 2 minutes. Drain buttermilk from bowl, add to buttermilk container, and refrigerate until ready to use. Knead salt, if using, into butter with wooden spoon. Transfer butter to second airtight container and refrigerate until ready to use. (Butter can be refrigerated for up to 2 months.)
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