For the curd strudel, first prepare the strudel dough.
Work all ingredients on a board until the dough separates from the board and hand. When the dough is completely smooth, place it on a floured part of the board and either coat it with oil or wrap it in plastic wrap lightly dusted with flour. After half an hour of resting, roll out the dough into a rectangle on a floured tea towel. Later, reach under the dough with the backs of both hands, stretching each one from the center to the edge until it is paper-thin.
For the filling, cream the soft butter with sifted powdered sugar, lemon zest and vanilla sugar. Mix in the egg yolks, strained quark and sour cream. Whip the egg whites with the granulated sugar, fold into the curd mixture and mix in the flour.
Brush the strudel dough with melted butter. Spread 2/3 of the dough surface with the filling. Sprinkle with raisins and roll up. Place the strudel on a buttered baking tray, brush with liquid butter and bake in a heated oven at 170 degrees Celsius for about 35 minutes.