Grease an 8 inch (20 centimeter) long oval baking dish with butter.
If desired, add a thin layer of the raspberry or lingonberry sauce across the bottom of the baking dish.
Preheat oven to 400˚F (200˚C) with the rack in the center of the oven.
In a small bowl, lightly beat together the egg yolks until smooth. Set aside.
In another bowl, sift the flour, then add the lemon zest and salt. Set aside.
In the bowl of a stand mixer fitted with a whisk attachment or a large bowl using a hand mixer, beat the egg whites just until stiff peaks form.
While beating, slowly add the granulated sugar, then the vanilla sugar, until smooth and shiny.
Pour the beaten egg yolks over the stiff egg whites, then add the flour.
Use a rubber spatula to gently fold together the ingredients just until combined with no white streaks of egg whites remaining. Take care to not deflate the egg whites.
Using a silicone bench scraper or large rubber spatula, arrange the fluffy egg mixture into three piles in the prepared baking dish. Use the spatula to gently shape each pile a mountain peak.
Bake in the preheated oven until golden brown across the tops. It should still be creamy on the inside. Do not open the oven door while baking or the mountains will deflate.
Serve the Salzburger Nockerl immediately from the oven with a dusting of powdered sugar and additional raspberry/lingonberry sauce on the side if desired. The mountain peaks will quickly start to deflate after removing from heat.