Bakers often wonder about "hollow macarons." There seems to be 2 distinct types of holes in macaron shells.
1. Air pocket at the top, under the dome of the macaron. I believe this is primarily due to excess air in the batter after folding. Either fold it a bit longer, or simply let the batter sit 15 minutes - 1 hour before piping (in the bag, so it won't dry out), and/or slam the sheet pans a bit more (check out our video for how to do this). This type of air pocket could be affected if the egg whites are freshly cracked -- they might need more aging and/or powdered egg white to reinforce the meringue thru strenegthened egg proteins... Sometimes I can tell a macaron will have this type of air pocket because in the final minutes of baking, you can almost see the skin wrinkling... Letting the baked macarons sit on their sheetpans 24 hours can also help minimize the air pocket.
2. Air pockets at the bottom, where the macaron baked against the silicone sheet or parchment on the sheetpan. This seems to be most affected by the pressure when the batter is squeezed thru the pastry bag. I have seen 2 people pipe the same batter on the same sheetpan and bake it in the same oven for the same amount of time -- and all other things "equal," the only variable is strength of squeezing. Basically, you are mimicing a depositor used by the largest macaron manufacturers to compress the batter thru a very tiny hole and deposit like 56 macarons in a few seconds... Squeeze hard. Very hard. Like crushing a tennis ball in physical therapy.
Any other questions? Please send a photo of the result from your baking (and I can troubleshoot "my" recipes better but we could maybe get somewhere if you're having trouble with someone else's recipe... Email Kathryn