Cake Faults

This week's small batch of chocolate cupcakes turned out nicely. The previous two weeks, using a different recipe scaled down, did not. I read about cake faults to note that

Fault — cake sinking in the center

Causes

1. Too much aeration. This may be caused by:
(a) Too much sugar used in the recipe. This can be detected by excessive crust color and a sticky seam running in the shape of a U.
(b) Too much baking powder. Difficult to detect because it can be confused with (c).
(c) Overbeating of fat/sugar/egg batter prior to adding flour.

2. Undercooked. This can easily be detected by the presence of a wet seam just below the surface of the top crust.

3. Knocking in oven prior to cakes being set. If during cooking when all the ingredients are in a fluid state, a cake gets a knock or disturbance (such as a draught of cold air) some collapse may take place which will result in the center of the cake caving in.

4. Too much liquid. This is easy to detect because, firstly the sides will tend to cave in as well as the top, and if the cake is cut a seam will be discovered immediately above the bottom crust. Cakes containing too much liquid do not show this fault until they are removed from the oven. During baking, the excess moisture is in the form of steam and actually contributes to the aeration of the cake. On cooling, this steam condenses into water which sinks to the bottom of the cake, collapsing the texture by so doing.

I didn't see the behavior described for #4, so that's not it. I was careful not to knock them, so not #3 either. I really doubt #2 as well, just based on the clean toothpick. The cupcakes rose, and then fell, so I think it's #1. I don't think it's a because the cupcakes weren't as sweet as most. I didn't use any egg in the scaled down version, so either I did too much with the butter and sugar (by hand? really?), or too much baking soda (no baking powder in that recipe either). Or maybe it was leaving out the egg? Maybe more protein would hold the risen shape and not collapse?

Since the first recipe didn't scale down well, and I'm enjoying testing recipes designed for small batches, I don't think it matters. But it's nice to know what causes this sort of problem too.