Preheat oven to 300 degrees F. Line two cookie sheets with parchment paper and lightly spray them with nonstick spray.
Separate the eggs into two bowls, whites in a medium bowl and yolks in a large bowl.
Using an electric whisk, whip the egg whites and the cream of tartar until stiff.
Whisk together the egg yolks, Greek yogurt and salt, until smooth.
Using a spatula, carefully fold the egg whites into the yolk mixture, working in batches. Work by placing a mound of egg whites on top of the yolk mixture, then gently fold the yolk mixture from under and over the egg whites. Fold again and again until the mixture is incorporated.
Using a ¼-cup ice cream scoop or measuring cup, spoon 5-6 large mounds of the mixture onto each of the prepared baking sheets. Gently press with a spatula to slightly flatten.
Bake until golden brown and set, about 30 minutes.
Cool a couple of minutes on the cookie sheet, then gently transfer the cloud bread to a wire rack to cool completely. You can also use them while still warm.