Why do my macarons have a proper shape and feet, but are hollow inside?
If they are hollow inside, it can also be because they have been cooked too long and/at a temperature that was too high. Most of the recipes say it should be between 140-155° F for 10 to 15 min, but then it depends on your oven, the size of your macarons, and a 1000 little details in the way you made them, so the best is to try little by little to figure out your own ideal temperature!