To make royal icing: Combine the egg white and powdered sugar in a medium-size mixing bowl and whip with an electric mixer on medium speed until opaque and shiny, about 5 minutes. Add the lemon juice and continue whipping until completely incorporated, about 3 minutes. The lemon juice whitens the royal icing. The royal icing should be light, fluffy, and slightly stiff. You may need to adjust the consistency by adding more egg whites if the icing is too dry or more powdered sugar if it is too wet.
Place the royal icing in a small piping bag or paper cornet.
The Cornet: A cornet is a small piping bag made from parchment paper. It is usually used to make fine decorations.
Cut an 8-by-12-by-14 1/2-inch triangle from a sheet of parchment paper. Hold the middle of the long side of the triangle between two fingers of 1 hand. Take the tip of the triangle on the short, wide end and roll it toward the other tip of that same end while simultaneously pulling it in an upward motion. The tip of a cone will form where your thumb and finger hold it on the long side.
Release your grip from the long side, so that you are now holding the 2 corners where they meet. The paper will already resemble a partially formed cone. Roll the remaining tail until it is completely rolled into a cone. There will be 1 point sticking up from the open end. Fold it inside toward the center, and crease the fold. Now you should have a cornet. To close the cornet once it has been filled, fold it away from the seam; this will keep the seam from opening. Use a pair of scissors or a sharp paring knife to cut an opening at the tip of the cornet to the desired size.
Consumption of raw or undercooked eggs, shellfish and meat may increase the risk of foodborne illness.
Recipe courtesy of "How to Make a Cornet: From Dessert Circus, Extraordinary Desserts You Can Make At Home" by Jacques Torres