Why did my macarons deflate in the last 4 minutes of baking?

Question

I've been learning how to make macarons using the Bravetart/Stella Parks recipe (archived on Wayback Machine). Here's what happened in my most recent attempt:

1. When piped out on the sheet, the batter stayed somewhat mounded up (so I don't think they are overmixed, or at least, not as much the last time I did it!).
2. When in the oven in the early stages of baking, they had some nice height to them, though no feet at any stage, as shown in this image (around 5-7 minutes in):
3. However, after taking them out of the oven, they were totally flat---only about 4mm thick. (I did bake for a few extra minutes since they had already deflated and seemed very squishy still). The surface is rough, and looks like it is full of small holes, as shown in this image of them still on the tray:

The same thing happened for every tray I baked, so for the second tray I watched the whole time & saw that they "deflated" with about 3.5 minutes left on the timer (so, after they had been in the oven about 14.5 minutes).

Other possibly relevant details:

• I have an oven thermometer, so I know the oven was indeed actually at 300 degrees
• I have a stand mixer, so I was able to follow the timing & speed directions of the recipe exactly when making the meringue. I indeed ended up with a very stiff & dry meringue.
• I do not have a food processor, but I used pre-ground King Arthur almond flour, and used a blender for 30 seconds in batches to grind it a little finer with the powdered sugar. Not sure if this was necessary, but my sifter/sieve is very fine, and even with the extra grinding takes a little effort to get the mixture through.
• I knocked each tray a couple times on each side to pop bubbles. My last tray I let sit for 30 minutes after knocking, and I noticed more bubbles had developed, so I of course knocked it again to pop them (but, it turned out the same as the rest).

Answer 1

Caveat: I'm not much of a baker and I'm going off my memory of my/my wife's attempts to make them quite a few years ago, which almost all ended up like yours...

I would say that the steam which causes them to rise has managed to escape - there are a few ways this can happen and most of them are caused by bubbles in the mixture.

1. Don't gently knock from the side, give that tray a firm sharp tap down flat on a bench/table to get rid of the bubbles.

2. Don't pipe them too big - too big and they aren't strong enough to support the dome themselves and collapse. 1.5-2 inches (3.75-5 cm) is about as big as you want to go. A template will help with this.

3. The top after piping should form a smooth dome (not an irregular mound = too dry) shortly after being piped and not spread outside the template (=not stiff/dry enough).

On the plus side; flat ones are still super tasty and deliciously chewy, so have fun and don't worry too much about failures when trying to master this.