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!

I agree with Sarah, temperature can be a factor. Many ovens are off slightly so if you believe the oven was set to the correct temperature it might be worth investing in an Oven Thermometer. You can find some for under five dollars (USD). It will help you gauge the internal temperature of your oven so you'll know if it is off. If it is off you'll just want to adjust the temperature you set your bakes for. So for example if you have a recipe that calls for a temperature of 350° F but your oven is running 10° F over, set the temperature for 340° F and that should hopefully solve the problem.