Why a sponge comes out domed — the cause
A flat sponge is half the battle with a bento cake: it’s easier to assemble, holds its shape better and looks tidier on the cut. But for beginners the layer often rises into a peak in the middle.
What happens in the oven
The walls of the tin heat first. The batter around the edge sets and forms a rigid rim — while the centre is still liquid. It has nowhere to go but up. Hence the cone in the middle, and sometimes a crack across the top.
The higher the temperature and the smaller the tin, the stronger the effect. At bento diameters of 10–12 cm it shows up especially clearly: a small tin heats fast at the edges, while the centre needs the same time it would in a large one.
What it tells you
A dome almost always means one thing: the oven is too hot for this size of tin. Not “the wrong recipe”, not “bad flour” and not “weak baking powder”.
Domestic ovens routinely lie by 10–25 degrees, in both directions, and gas and electric behave differently. So the temperature that gives you a flat layer is always a pair — “this specific sponge plus this specific oven” — never a single number off the internet.
Why it matters more for bento
A bento cake is 6–8 cm tall, which is only 2–3 layers. Every dome has to be trimmed, and that’s 2–3 mm of usable height gone from each layer. On a large cake the loss is invisible; on a bento it eats the proportions.
If you’re trimming a tall dome every single time, that’s no longer cosmetic — it’s product loss. More on proportions in the piece on bento cake size.
We don’t publish temperatures, times or the order of steps for individual sponges on the blog. All ten recipes in our PDF collection carry exact baking settings, gram weights for 1, 2 and 3 cakes, and separate notes for the sponges where doming is the typical problem. That’s the part of the work the collection is there for.