A sponge cake that uses colored batter to add a decorative flair. It can be filled with mousse, ice cream, whipped cream, or anything else you desire. The sponge batter recipe is adapted from Rose Leavy Beranbaum's "Biscuit Roulade" recipe in her book, The Cake Bible.
Cream together butter and powdered sugar with an electric mixer until well combined. Add egg whites and beat well.
Stir in flour (or flour and cocoa powder if making chocolate).
Divide batter and color with food colors. Place batter in icing bags fitted with small round tips, or place in plastic sandwich bags and trim small end off corner.
Draw image on parchment paper that is cut to size for a 17x12" jelly roll pan. Place parchment upside down on baking sheet and pipe colored/chocolate batter on image. Freeze for 10 minutes in between layers.
When image is complete, freeze for 1 hour before adding sponge batter.
Preheat oven to 450° F. Whisk together cake flour and cornstarch. Sift through a sieve then set aside.
Separate 2 of the eggs, placing the yolks in the bowl of a stand mixer fitted with a paddle attachment, and the whites in a different bowl. To the yolks, add the additional yolk, the 2 remaining eggs, 1/2 cup sugar and food coloring, if desired. Beat on high speed for 5 minutes until thick, fluffy and triple in volume. Beat in vanilla.
Sift half of the flour mixture over the egg mixture and fold it in gently with a large whisk. Repeat with the remaining flour mixture. At this point, if you only have one mixer bowl, transfer the batter into a different bowl and wash out your mixer bowl very well so that no traces of grease remain.
Transfer the egg whites into the grease-free mixing bowl and place in the stand mixer, fitted with a whisk attachment. Beat until foamy then add cream of tartar and beat until soft peaks form. Add the remaining 1 Tbsp of sugar and beat until stiff peaks form. Fold the whites into the batter.
Remove the pan from the freezer and use butter or shortening to grease only the sides of the pan. Begin by carefully pouring batter over the colored image, then pouring it over the rest of the pan. Use an offset spatula to carefully spread the batter, being extra careful around the image so you don't smear the colored batter.
Tap the pan on the counter a few times to remove some of the air bubbles. Bake in preheated oven for 7 minutes, or until golden brown and springy to the touch.
Immediately loosen the edges from the sides of the pan with a sharp knife. Lay another piece of parchment paper over the cake and lay another pan on top of the cake then flip it over and remove the hot pan. Carefully lift the parchment off of the cake.
You can either use the cake as a Swiss roll, or cut it to the desired size and use it flat, or form it in a mold like I did. Refer to the tutorial for instructions on forming it in a mold. Be sure to roll it or form it while still warm but allow to completely cool before adding a filling.