Preheat the oven to 300° F. Line a baking pan or molds with parchment paper and lightly spray with nonstick spray. Separate the eggs, making sure no yolk gets into the whites. Whip the egg whites and the cream of tartar until stiff. In a separate bowl, whisk together the yolks, mascarpone cheese, salt, and stevia until smooth. Using a spatula, gradually and carefully fold the egg whites into the yolk-cheese mixture. You want to use the folding technique because you don't want to lose the fluffiness of the egg whites. Spoon 4 large mounds of the mixture onto the prepared baking sheet or molds. Gently press with a spatula on the top of each mound to flatten just slightly.