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The batter needs to be thick enough to hold the jam in place, so mix the flour, baking powder and salt in only briefly - overmixing loosens the structure and lets the jam slide down. Drop the jam in small teaspoon-sized dots on the surface rather than pouring it in, and swirl with just one or two passes of the skewer. Digging deep with the skewer pulls the jam straight to the base.

Yes, fold them into the batter at the end instead of swirling jam on top, and toss them in a little of the measured flour first so they stay suspended. Keep in mind that fresh fruit brings more moisture than 3 teaspoons of jam, so the loaf may need the upper end of the 40-45 minute range. Use frozen berries straight from the freezer without thawing, or they will bleed into the batter.

Sour cream, crème fraîche or buttermilk all work in the same 1 tablespoon quantity - they add the same slight acidity and moisture that keeps the crumb tender. Since it is only one tablespoon, the loaf will still bake through if you leave it out, but the texture will be a touch drier. Fold whichever you use in gently at the end, as the recipe does, rather than beating it in.

Let it cool completely before wrapping, otherwise trapped steam makes the surface sticky and the jam swirls damp. Kept airtight at room temperature it stays soft for a few days - the butter and yogurt help here. For longer storage, freeze it whole or in slices, well wrapped, and dust with powdered sugar only after thawing, since it dissolves on a frozen surface.

Weigh out the flour, baking powder and salt in their bowl in advance and grease or line the 10 x 20 cm pan, so you only have the creaming and mixing left. Don't let the finished batter sit, though: the baking powder starts working as soon as it meets the yogurt and eggs, so get it into the pan and the preheated oven right away. Taking the butter and eggs out early so they reach room temperature also makes the creaming step much faster.