Preheat your oven to 330°F (160°C). Line a 7×11-inch baking dish with parchment paper.
Separate the egg yolks from the egg whites, ensuring no yolk mixes with the whites. This is crucial for achieving stiff peaks when beating the egg whites.
In a bowl of an electric mixer, beat the egg whites until stiff peaks form. Transfer the beaten egg whites to another bowl.
In the same mixer bowl, add the monkfruit sweetener to the egg yolks along with the vanilla extract (if using). Beat the mixture until the eggs become light and fluffy.
Add the Greek yogurt to the egg yolk mixture and mix until fully incorporated.
Sift the cornstarch into the egg and yogurt mixture, combining until smooth.
Carefully fold the beaten egg whites into the yogurt and egg batter, maintaining as much air as possible for a light and fluffy texture.
Pour the batter into the prepared baking dish. Place this dish on a larger baking sheet.
Fill the baking sheet with boiling water, enough to submerge the baking dish by 1/4 to 1/3.
Bake at 330°F (160°C) for 45 minutes. Then, raise the oven temperature to 350°F (180°C) and bake for an additional 15 minutes.
Once baked, turn off the oven. Crack the oven door open and allow the cake to cool inside for about 1-2 hours before removing it.