Sift flour into a large bowl, making a well in the center. Add 1/2 white cup sugar, egg, and vanilla sugar into well. Distribute 1/2 cup softened and 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 prepared pan, smoothing out any bubbles. Cover with rhubarb.
Crumble remaining dough between your fingers and sprinkle over rhubarb. Sprinkle with remaining 3 tablespoons white sugar, then scatter 2 tablespoons cold cubed butter over streusel.
Bake in the preheated oven until streusel is lightly brown and crisp, 40 to 50 minutes.
