What is the main ingredient in macarons?

A macaron is a sweet meringue-based confection made with egg white, icing sugar, granulated sugar, almond meal, and food colouring.

Egg whites are necessary in order to create a meringue, into which the almond flour and powdered sugar are folded into to create the batter. The lower the egg white content, the less moisture in the macaron batter. Making the meringue is a really important step in the macaron making process, when you make macarons, you want the meringue to have smaller, more stable air bubbles.

A macaron is made by combining icing sugar and ground almonds into a fine mixture. In a separate bowl, egg whites are beaten to a meringue-like consistency. The two elements are then folded together until they are the consistency of "shaving foam", and then are piped, left to form a skin, and baked.

Sometimes, a filling is added such as buttercream or clotted cream, ganache, or jam.