A sponge can collapse five different ways, and each one points at a different cause.
Find yours below rather than working through a general list.
It sank in the middle, edges fine
Underbaked. The centre never set, so it fell as soon as the support of the oven heat
was gone.
The skewer test lies in a very moist batter. Better cues: the top springs back when
pressed lightly, and the cake has just begun to pull away from the sides of the tin.
It dropped suddenly while baking
The oven door. Until the protein sets, a sponge is a foam, and a rush of cold air makes
it fall. Do not open the door for the first two thirds of the bake.
It never rose much at all
The air was stirred out. All the lift in a classic sponge comes from beaten eggs,
so the flour is folded in — cut down through the middle, sweep along the bottom,
turn the bowl, stop as soon as no dry flour shows.
It rose high, then sank as it cooled
Oven too hot. The crust set before the middle finished rising, so the structure was
never properly built — it domed, cracked, and gave way on cooling.
Most domestic ovens run 10–20 degrees above the dial. This is the fault an oven
thermometer solves permanently.
It sank after leaving the tin
Turned out too soon. Cool in the tin for ten minutes, then on a rack.
A hot sponge collapses under its own weight.
The one purchase worth making
An oven thermometer. Two of the five causes above are temperature, and no amount
of technique compensates for an oven that is not the temperature it claims.
See also:
Why sponge cake is dense ·
Why cake cracks on top ·
Why egg whites will not whip ·
Cherry Almond Pie