01 - Preheat oven to 300°F (150°C) and line a baking sheet with parchment paper.
02 - In a large clean mixing bowl, beat the egg whites and cream of tartar with an electric mixer until soft peaks are formed. Gradually incorporate the granulated sugar, continuing to beat until stiff and glossy peaks form.
03 - In a separate bowl, whisk together the egg yolks, cornstarch, and vanilla extract until the mixture is smooth.
04 - Gently fold the egg yolk mixture into the whipped egg whites, taking care not to deflate the volume.
05 - Portion the combined batter evenly into separate bowls according to the desired number of colors. Add a few drops of each gel food coloring to its respective bowl and delicately fold to ensure even color distribution.
06 - Spoon alternating dollops of colored batter onto the prepared baking sheet to create a rainbow effect. Lightly swirl with a spatula for a marbled appearance if preferred.
07 - Bake in the preheated oven for 25 minutes, or until set and lightly golden.
08 - Allow to cool briefly before gently removing from the baking sheet. Serve immediately for optimal texture.