01 - Preheat oven to 300°F (150°C) and line a baking sheet with parchment paper.
02 - In a clean bowl, beat egg whites and cream of tartar with an electric mixer until soft peaks form. Gradually add sugar, continuing to beat until stiff, glossy peaks develop.
03 - In a separate bowl, whisk together egg yolks, cornstarch, and vanilla extract until smooth.
04 - Gently fold the egg yolk mixture into the whipped egg whites, maintaining volume.
05 - Divide the batter evenly between bowls, one for each desired color. Add several drops of gel food coloring to each batch and fold carefully until combined.
06 - Spoon the colored mixtures side by side onto the prepared baking sheet. Swirl gently with a spatula for a marbled, rainbow effect if preferred.
07 - Place the baking sheet in the oven and bake for 25 minutes, until set and lightly golden.
08 - Allow to cool for several minutes on the sheet, then serve immediately for optimal fluffiness.