I went through a lot of batches of hollow macarons before I finally figured out the problem — I hope I did, anyway!
I started out by doing thorough search on the internet of course but then came back to my most reliable sources: http://bravetart.com/blog/HollowPursuits and http://www.eatlivetravelwrite.com/2012/03/why-do-my-macarons-have-hollow-shells-a-work-in-progress/
I was already resting the macarons for about an hour before I popped them in the oven — just until I could glide my finger over the surface. I did start using fresh egg whites left at room temperature for a few hours — this did make it easier to whip up a stiff meringue in less time but I was still getting hollows.
Back to my notes. I noticed that I started getting hollows when I switched to Earl Grey macarons from the plain macarons. I hadn’t been grinding the tea in a spice grinder and I wondered whether my dry ingredients were just too heavy for the macarons. So I left out the Earl Grey tea leaves– better. Then I started using a spice grinder and sifting more — much better. Just little air pockets now.
Then about this time we got a Trader Joe’s in Orlando. Now I had refused to buy the pre-packaged almond flour from my supermarket up to this point because it was $12/lb. But Trader Joe’s has it for about a third of that price so I bought two pounds (because that stuff flies of the shelves) and made another batch. Holy Macaron! Perfect! Full inside, perfect outside! FINELY GROUND ALMOND MEAL WAS THE KEY!!!
AND NOW HERE IS MY REVISED LIST OF MUST DO’S FOR MACARONS:
1) Absolutely positively buy prepackaged ground almonds or grind them at home into a powder in a spice grinder.
2) Use real egg whites not liquid, leave them at room temp for a few hours
3) Grind any other ingredients you are adding to the basic recipe — such as freeze-dried raspberries, tea leaves– in a spice grinder
4) Test the consistency of your macaronnage by dropping a small amount on a plate and verifying that the peak falls back into the “blob” within 10 seconds
5) Dry your macarons until you can glide your finger over the surface of the macaron
6) Use your convection fan if you have an unreliable oven with hot spots