01 - Preheat oven to 300°F. Line a baking sheet with parchment paper.
02 - In a large clean bowl, use an electric or stand mixer to beat egg whites and cream of tartar until soft peaks form. Gradually add sugar and continue beating until glossy, stiff peaks develop.
03 - In a separate bowl, whisk together egg yolks, cornstarch, and vanilla extract until the mixture is smooth and lump-free.
04 - Gently fold the egg yolk mixture into the beaten egg whites, taking care to preserve as much volume as possible.
05 - Divide the batter evenly into separate bowls, one for each color desired. Add a few drops of gel food coloring to each bowl and gently fold until the color is evenly distributed.
06 - Spoon colored batter onto the prepared baking sheet in adjacent dollops to form a rainbow effect. Optionally, use a spatula to gently swirl and marble the colors.
07 - Bake in the preheated oven for 25 minutes, or until set and lightly golden.
08 - Allow to cool briefly on the baking sheet before serving for maximum fluffiness.