Thin, ornate sponge sheets with striped patterns in various configurations can be made very easily using Silpats (imported French silicone baking mats) and a special scraper that is made to fit the mats. This tool, called a decorating comb, consists of a frame and interchangeable blades (typically made of rubber) that have notched edges and are available in different sizes and patterns. A similar but less expensive tool is a molded plastic comb with two patterns, one on each side. These are intended for chocolate work but can be used to create sponge sheets as well. Because the plastic combs are smaller, it is necessary to make two passes over a full sheet instead of one. For more information, see “Decorating Comb with Frame”.
The two recipes for Joconde Sponge Base that follow are fairly similar and can be used interchangeably. Sponge Base II takes a bit longer to make but is a better choice if you plan to add coloring to the base portion. With either recipe, be sure to keep the whipped egg whites and sugar very soft because a thinner batter will flow out and fill in much more tightly between the tuile paste lines. Being careful not to overwhip the egg whites will also help prevent air bubbles in the finished sponge sheets, which can detract from the appearance.