"Rhubarb cake (Rhabarberkuchen) is very popular in Germany and this version is especially delicious. It is fairly quick and easy to make since the dough for the crust and the streusel are the same. Try it! You won't regret it."
Sift flour into a large bowl and make a well in the middle. Add egg, 1/2 cup sugar, and vanilla sugar to the well. Distribute 1/2 cup cubed butter evenly on top. Mix into a dough using your hands. Cover and refrigerate for 30 minutes.
Preheat the oven to 325 degrees F (165 degrees C). Grease a 9-inch springform pan.
Place 2/3 of the dough in the springform pan and smooth across the bottom. Cover with rhubarb.
Crumble remaining dough between your fingers and sprinkle over the rhubarb. Sprinkle with 3 tablespoons sugar. Scatter 2 tablespoons cubed butter over the streusel.
Bake in the preheated oven until streusel is lightly brown and crisp, 30 to 40 minutes.