PREHEAT oven to 425F, then line a baking sheet with parchment.
SEPERATE egg whites from yolks into a large bowl. Keep yolks intact in a smaller bowl. Beat egg whites and salt with an electric mixer on high until stiff peaks form, about 3 min. Fold in parmesan. Divide mixture into 4 mounds about 3 in. wide on prepared sheet. Use the back of a spoon to make wells in centre of mounds.
BAKE for about 2 min. Remove from oven and add a yolk to each well. Continue baking until whites are firm but yolks are still soft, about 3 more min. Serve on toast, if desired.