You need to dry out your macarons for enough time before baking them. The drying will form a  layer on the top and will not allow the expanding air to escape. If the layer on top is not thick enough, it will burst, causing the air to escape and forming cracks on the top of macarons. Dry out your macarons for 8 to 9 hours before putting them in the oven. You can also dry them in a food dehydrator for 20 minutes on the lowest temperature setting.