Here’s Stella Parks’ troubleshooting guide on hollow shells.
1. If you want to understand why your macarons turn out hollow, skip the flavoring and food dye. To get to the bottom of hollows, you have to divorce the problem of ingredients from the problem of technique.
If that seems boring, grab some watercolor brushes and paint the finished shells with food coloring, grab a food-pen and doodle on them, and use a stencil and some cocoa powder to pattern them, rub the baked shells with luster dust or go wild with a flavorful, colorful filling.
2. Underbaked macarons have a soft interior that doesn’t have enough structure to support itself as it cools, so make sure the macarons have baked through and will cleanly peel away from the parchment paper. I don’t like baking macarons on silicon because even underbaked macarons will slip off easily, which makes judging doneness more tricky and hollows more common.
3. Crack one macaron open before you pull the rest out of the oven. Is it already hollow? If so, the problem probably goes back to the meringue or macaronage, but it could mean your oven runs a little hot. Forget what it says on the dial, check the temperature with an oven thermometer.
4. If you crack that macaron open and find out it isn’t hollow, do a flip test! As soon as the macarons come out of the oven, lift up the parchment and carefully turn it upside down with all the macarons still attached. Set aside one or two macarons to cool right side up. Once they’ve fully cooled, break open one of each.
Either both macarons turn out perfect (high five!), they both turn out hollow (aw, shucks!), or you wind up with mixed results. If the upside-downers have a nice structure and the right-side-uppers are hollow, the problem is that the interior is collapsing as it cools. Why is another matter.
One reason may be that the macarons are cooling too fast, which can happen in the wintertime or if you’ve got the AC set to Frosty the Snowman (I found this out my first January in the Pastry Dungeon). Another reason may be the macaron is slightly underbaked and, like an angel food cake, will benefit from cooling upside down. This helps the macaron maintain its structure until it sets firmly enough to support itself.
5. Don’t skip sifting even with pre-ground almond flour and don’t skimp on grinding for whole nuts. When sifting, use a sieve not a colander. If a large amount of almond meal will not pass through the sieve, put it back in the food processor and keep grinding and sifting until more than 2 tablespoons of chunky bits remain. Too many small chunks make an abrasive dry mix that will tear holes in the meringue as it tries to rise in the oven and cause collapsed, hollow macarons.
6. Try a splash of oil. Macarons are fairly lean by nature and sometimes a little
extra fat is all they need. Any odorless, flavorless oil will work. Add in 4 teaspoons of oil along with the dry ingredients before grinding and/or sifting. Don’t worry about the slight dampness, just sift as usual; use a spatula or whisk to make sifting easier if you need to. The oil seems to minimize hollows in a variety of circumstances, but most especially when using less-than-fresh nuts (or nuts without an expiration date, like some purchased from a bulk bin).
I know from countless impassioned emails that preventing hollows is a major concern, but please give yourself time and space to learn. Macarons involve too many variables for anyone to reasonably expect to master them in only a handful of attempts. Don’t forget that macaron are for eating and even the best pastry shops in Paris sell hollow, funky, mismatched macarons.
Have fun. Enjoy the process of making them and learning. Share them with friends and family; they’re not judging you. And if they are? They don’t deserve your delicious macarons (jerks). Because with or without hollows, macarons will always be delicious.