Beat the egg yolks with sugar until foamy. Add flour mixed with cocoa and finally fold in the whipped egg whites. Place on a baking sheet lined with parchment paper and bake at 170°C for about 8 minutes. Transfer the baked roll to a damp towel and roll it up while still warm. Prepare the filling by blending cottage cheese and butter. Flavor with sugar and vanilla sugar. Spread the filling on the cooled roll, roll it up, and let it chill in the refrigerator. Dust with powdered sugar or drizzle with melted chocolate.