01 - Preheat oven to 300°F and line a baking sheet with parchment paper.
02 - In a large, clean mixing bowl, beat egg whites with cream of tartar on medium-high speed until soft peaks form. Gradually add the 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 becomes smooth and uniform.
04 - Gently fold the egg yolk mixture into the whipped egg whites with a spatula, preserving the airy texture.
05 - Divide the batter evenly among four bowls. Apply a few drops of desired gel food coloring to each portion and fold gently until combined.
06 - Spoon colored batter in dollops onto the prepared baking sheet, arranging side by side for a rainbow effect. For a marbled look, swirl gently with a spatula.
07 - Bake for 25 minutes, or until the surface is set and lightly golden.
08 - Allow to cool several minutes on the baking sheet before serving for optimal fluffiness.