1. Preheat oven to 325°F. Whisking by hand or using a heavy-duty electric stand mixer, beat egg whites and baking soda in a large bowl until stiff peaks form.
2. Stir together egg yolks and Greek yogurt in a small bowl. Fold yolk mixture and salt into egg white mixture.
3. Dollop egg mixture in 4 mounds on a parchment-paper lined baking sheet very lightly coated with cooking spray, and spread each into a 4-inch round. Bake in preheated oven until lightly golden, 12 to 15 minutes. Serve with desired toppings.