Butter is made by continuing to whip cream. The cream first foams, then stiffens, and at one point suddenly separates into two parts: yellow butter granules and a whitish liquid. Strain off the liquid, wash the remaining butter in ice water, and squeeze out the last liquid by hand. If you skip the washing step, the butter will turn sour within a few days.
The fat in cream is dispersed in water as tiny droplets, each surrounded by a thin membrane. Whipping damages these membranes, and the droplets begin to stick together. Up to a point, this process traps air bubbles and produces whipped cream. If you keep whipping, the droplets merge completely, the foam collapses, and the fat separates from the water. So overwhipping whipped cream is not a mistake; it is actually the first step toward making butter. If cream is accidentally overwhipped, do not throw it away; whip it for a few more minutes and turn it into butter.
The starting ingredient matters. Use cream with at least 35% fat; low-fat cream will not produce butter granules even if whipped. Temperature matters, too: ice-cold cream separates slowly, while lukewarm cream produces overly soft granules. The easiest temperature to work with is when the cream is cool but not ice-cold; taking it out of the refrigerator and letting it sit briefly is enough. Pasteurized cream can also be used; the fat content, not the processing, determines the result.
Whip with a mixer or by shaking a lidded jar. The mixture successively foams, rises, stiffens, takes on a matte appearance, and finally separates all at once into 2 parts: yellowish butter granules and a whitish liquid. Stop the mixer at this stage, or the liquid will splash everywhere. Choose a deep bowl and do not fill it more than halfway. If using a jar, fill it halfway and keep shaking without stopping. When using a mixer, covering the bowl with a cloth also prevents splashing.
Strain off the separated liquid and do not discard it; it is a buttermilk-style product that can be used in pancakes, savory pastries, or cake. Transfer the butter granules left in the strainer to a bowl. Now comes the most commonly skipped step: wash the butter with ice water. Pour off the water, add fresh water, and repeat the process several times; continue until the water is clear. The washing water must be ice-cold; lukewarm water softens the butter and the granules fall apart.
Washing is for storage. Milk residue left in the butter speeds souring; homemade butter that has not been washed well changes flavor within a few days. Press the washed butter with a spoon or by hand to remove the last liquid, salt it if desired, wrap it in paper, and refrigerate it. One liter of cream yields about 400 grams of butter; the ratio varies with the cream’s fat content. For longer storage, divide the butter into portions and freeze them, then take out one portion as needed. If adding salt, use fine salt and knead until it is evenly distributed through the butter.