Butter is fat suspended in water, held apart by protein. When you churn cream, the fat globules clump and the water separates out as buttermilk. Culturing the cream first, letting a yogurt or buttermilk culture work on it for a day, gives the finished butter a tang and depth that sweet butter does not have. You will taste the difference immediately. ## The short version Let 600 ml whole-milk cream sit with a spoonful of live yogurt at room temperature for 24 hours. Chill. Whisk in a stand mixer until the butter breaks from the buttermilk. Squeeze out the liquid, rinse the butter in cold water, salt it, shape it. ## What you need - 600 ml whole organic cream, ideally 35 to 40 percent fat (skim the top off a jar of whole milk over a few days, or buy double cream)

• 2 tbsp plain organic skyr (as the culture, any live-culture dairy works, buttermilk is classic)
• 1/2 tsp fine salt (or flaky, to taste)
• Ice water ## How to make it 1. Culture the cream. Pour the cream into a clean jar. Stir in the skyr thoroughly. Cover loosely and leave at room temperature for 18 to 24 hours. It will thicken and smell lightly tangy, like crème fraîche.

2. Chill. Move the jar to the fridge for at least 2 hours. Cold butter separates more cleanly than warm.
3. Whip. Pour the cold cream into the bowl of a stand mixer with the whisk attachment. Start on medium. It will go through four stages: soft peaks, stiff peaks, whipped cream, and then, suddenly, grainy. That is the break.
4. Watch the break. Once the cream looks grainy and you see pale yellow solids flecking out of liquid, slow the mixer to low. The solids are the butter. The liquid is the buttermilk. Keep mixing until the two separate fully, about 30 seconds more.
5. Drain and squeeze. Pour the mixture through a fine sieve set over a bowl. Save the buttermilk for pancakes or bread. Gather the butter into a ball with your hands, squeezing out as much remaining liquid as you can.
6. Rinse. This is the step most people skip and most people regret. Put the butter in a bowl of ice water. Squeeze and press it until the water runs clear. Any trapped buttermilk will make the butter go rancid in a few days; rinsed butter keeps for weeks.
7. Salt and shape. Pat the butter dry. Knead in the salt. Roll into a log in parchment paper or press into a ramekin. Refrigerate. ## Why it works The culture lowers the pH of the cream, this is what gives cultured butter its tang. It also breaks the fat membranes slightly, so the fat globules cluster more readily when you churn. You get butter faster, with more flavour, and the buttermilk by-product is genuinely sour buttermilk rather than the sweet kind you get with uncultured cream. ## Notes - How much do you get? About 280 to 300 g butter from 600 ml cream, plus roughly 250 ml buttermilk.

• Keeps how long? Two to three weeks in the fridge if you rinsed it properly. Freezes well, 3 months.
• Salt level. Start with 1/2 tsp and taste. Unsalted butter is useful for baking; salted is better on bread.
• Troubleshooting. If the cream goes past stiff peaks without breaking, keep going. It will break. Do not add more cream.
• Pairs with. Rye bread. Any rye bread, but especially the one at the top of this list.