Zitronenmuffins

The single tablespoon of yogurt is there for moisture, so sour cream, crème fraîche or plain full-fat yogurt all work in the same amount. Buttermilk works too, though it is thinner, so add it slowly along with the lemon juice. If you leave it out entirely, the muffins still bake, they are just a little drier.

No, that is the acid in the lemon juice reacting with the butter-and-egg mixture, and it is very common. Beat the eggs in one at a time as the recipe says, and make sure the butter and eggs are at room temperature so they emulsify properly. Once the flour goes in, the batter comes back together and the muffins bake up fine.

That is almost always overmixing after the flour goes in. Once you add the flour, baking powder and salt alternately with the lemon juice and yogurt, stir only until no dry streaks remain - a few lumps are fine. Gluten develops fast in a small batch like this, and it shows up as a rubbery crumb with holes.

Keep them in an airtight container at room temperature; the butter and yogurt keep the crumb moist for a couple of days. Glaze only the muffins you plan to eat soon, since the thin lemon glaze softens and can weep on standing. They also freeze well unglazed - wrap them individually, then thaw at room temperature and glaze fresh.

You can rub the sugar with the lemon zest and whisk the flour, baking powder and salt together hours in advance. Do not mix the finished batter ahead, though - the baking powder starts working as soon as it meets the lemon juice and yogurt, so the muffins will not rise as well. The glaze is best stirred together right before drizzling, as it stiffens as it sits.