01 - Preheat oven to 300°F and line a baking sheet with parchment paper.
02 - In a large, clean mixing bowl, beat the egg whites and cream of tartar until soft peaks form. Gradually incorporate granulated sugar, continuing to beat until stiff, glossy peaks are achieved.
03 - In a separate bowl, whisk together 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 batter.
05 - Divide the batter evenly among four bowls. Add a few drops of each gel food coloring into separate bowls and fold carefully to incorporate, ensuring vibrant distinct colors.
06 - Arrange colorful batter dollops side by side on the prepared baking sheet. Swirl gently with a spatula to achieve a marbled rainbow effect, if desired.
07 - Bake for 25 minutes, or until set and lightly golden.
08 - Allow bread to cool for several minutes before removing from baking sheet. Serve immediately for optimal fluffiness.