Koldskål (Cold Buttermilk Bowl)

Variations

• What thickens a custard when there is no heat. Crème brûlée sets with yolk alone, which is why it cannot be brought back once it goes too far. Crème caramel has the whites in it because it has to stand up out of the mould. Blanquette de veau stops just short of setting. Koldskål is made of the same things with no heat at any point, so acid, sugar and the fridge divide that work between them.
• Put next to šaltibarščiai the question is different. That Lithuanian soup uses no heat either, and its argument is why it does not split — all three causes of dairy splitting need heat. Koldskål's argument is what gives it body. Same class of ingredient, different question. Dovga is the third answer: stabilise the yoghurt with egg and flour and boil it outright.
• The kammerjunker is not a garnish. It is a hard twice-baked biscuit and it starts softening the moment it touches the liquid, which is why it goes on at the table. Last batch's stroopwafel is the exact inverse — there the syrup softens a crisp wafer on purpose. One phenomenon; here it is the failure and there it is the point.
• Without buttermilk: 600ml plain yoghurt let down with 400ml milk and an extra tablespoon of lemon juice. But the acidity a cultured buttermilk was fermented to — around pH 4.5 — is half of both the body and the taste, so milk soured with vinegar on the spot does not give the consistency.
• Without raw egg: leave the yolks out and take the yoghurt up to 300ml and you still have something drinkable. It is closer to a lassi, though; the silkiness is the yolk. If it worries you, use pasteurised eggs — tiramisu makes the same call.
• Fruit: strawberries are the default, but raspberries, stewed rhubarb and summer plums all work.
• From the early 1900s, and industrial since 1979: it appeared once buttermilk became common in Denmark in the early 1900s, went into supermarkets in 1979 and is now almost always bought. In summer it takes a whole shelf of the chilled aisle.
• Dessert or a meal: properly it is lunch or an afternoon thing on a hot day. It is filed here as a dessert, and its name and the way it is eaten are both a soup's.

Tips

• Keep the order. Yolks and sugar, then yoghurt, then buttermilk. Combining like consistencies first is the whole of it.
• Dissolve the sugar into the yolks. Added later it does not dissolve in a cold liquid and sinks to the bottom.
• Season harder than seems right. Cold pushes sweetness and salt backwards.
• Two hours is a minimum. Overnight and the body is certain and the vanilla comes back.
• Biscuits last, and completely cold.
• Glass or china rather than metal. It is acidic enough to taste of an aluminium bowl.
• Do not keep it. Raw yolk, never heated: eat it within two days of the fridge.