Preheat the oven to 350 degrees F (175 degrees C). Butter a 10-inch springform pan.
Combine butter, sugar, vanilla sugar, and salt in a large bowl and beat with an electric blender until smooth and creamy. Add eggs one at a time, beating well after each addition.
Sift together flour and baking powder and mix into the batter, 1/4 cup at a time. Pour batter into the prepared springform pan. Press rhubarb pieces into the batter and sprinkle sugar over the top.
Bake in the preheated oven until lightly browned, about 30 minutes.
Meanwhile, beat egg whites in a glass, metal, or ceramic bowl until foamy. Gradually add sugar, continuing to beat until stiff peaks form and meringue mixture is glossy. Remove cake from the oven and spread meringue mixture on top.
Continue baking until meringue is set and lightly browned, about 15 minutes. Allow to cool in the springform pan.