Four possible causes and the simple adjustments that will give you a flat, even bake.
You have followed the recipe to the letter. The cake looked perfect through the oven door. But when you take it out, the centre has sunk into a sad little crater. Sound familiar?
Cause 1: Too Much Raising Agent
It seems counterintuitive, but too much baking powder causes a cake to rise too quickly, then collapse under its own weight before the structure sets.
Fix: Measure raising agents with proper measuring spoons, levelled off. Never heap.
Cause 2: Underbaking
If the centre is still liquid when you remove the cake, it has nowhere to go but down. The outside looks done, but the inside needed more time.
Fix: Use the skewer test. Insert a skewer into the centre — it should come out clean or with just a few moist crumbs, never wet batter. If in doubt, give it five more minutes.
Cause 3: Opening the Oven Too Early
A sudden drop in temperature causes the rising cake to deflate before the proteins have set. The structure literally collapses.
Fix: Resist the urge to peek for at least the first 20–25 minutes. Use the oven light instead.
Cause 4: Overmixing
Excess gluten development from overmixing creates a dense, heavy batter that rises unevenly and often sinks in the centre.
Fix: Fold flour in gently and stop as soon as no dry streaks remain. Less is more.
When All Else Fails
If your sponge has already sunk, all is not lost. Trim the top level, fill with buttercream, and stack with confidence. Nobody will ever know. Or better yet — call it a trifle and serve it in a bowl.