Sponge, jam, rolled — and harder than it sounds

It splits down the middle. That is the thing that happens, and everything anyone knows about making a rulltårta is organised around preventing it.

A rulltårta is a thin sockerkaka (sponge), spread with jam or buttercream, rolled into a cylinder. Central European in origin, probably mid-nineteenth century, and a Swedish fika fixture for about a hundred years. It is one of the cheapest things you can put on a table and one of the few where the difficulty is entirely in the handling.

The rules, and what they are worth

This is worth being careful about. There is a confident mechanical explanation in circulation, involving protein coagulation and starch setting at particular temperatures, and no proper source appears to sit under it. The temperature figures come from blogs quoting blogs. So what follows is the practice, which is consistent across every serious baking source, undressed as science it does not have.

Roll it warm. A warm sponge bends. A cooled one has set into the shape it cooled in, and forcing it into a different shape breaks the surface. This is the single rule that matters.

Roll it before it is filled. Turn the sponge out onto sugared paper, roll it up loosely with the paper inside while it is still hot, and leave it to cool rolled. It cools into a curve. When you unroll it, fill it and roll it again, it is going back where it already wants to go.

Do not overbake it. A dry sponge cracks and a slightly underdone one does not. Rulltårta is baked hot and fast, and a minute is a lot.

Replace some of the flour. Up to about a third of the wheat flour swapped for cornflour or potato flour gives a more flexible crumb. All potato flour is a step too far and cracks readily.

The day

Rulltårtans dag is 9 August, and unusually for these days somebody can tell you exactly where it came from. Maria Erdmann started it in 2012 with her husband and her brother-in-law, first as a Facebook group at the end of July that year. She has explained the date since: August is holidays, it is the middle of the berry season, and a rulltårta is an easy thing to ask people to bake.

That last reason is why it works. Most of these food days were created by an industry to sell an ingredient. This one was created by three people who thought it would be nice, and it is the best-documented day in the whole Swedish baking year.

One thing everybody repeats and nobody can source, and it belongs here as an impression rather than a fact: the rulltårta is often called the first cake a Swedish child learns to bake. No evidence for that turns up anywhere. It is plausible, given how few steps there are and how little it costs to get wrong, and an impression is still what it is.