Preheat oven to 338 degrees Fahrenheit (170 degrees Celsius). Butter and flour a 7 x 10.5-inch (18 x 28 cm) baking dish.
In a large bowl, whisk together eggs, sugar, salt, and vanilla sugar until light and fluffy.
In a separate bowl, whisk together flour and baking powder.
Add dry ingredients to wet ingredients alternately with warm water, mixing until well combined.
Pour batter into prepared pan.
Bake for 25 to 30 minutes, or until a toothpick inserted in the center comes out clean.