For the crust, throw all ingredients in the food processor and mix. Form a disk, wrap in plastic wrap, and refrigerate for at least an hour.
For the pastry cream, whisk egg yolks, half the sugar, and cornstarch until creamy. Boil milk with vanilla bean and remaining sugar. Gradually mix hot milk into egg mixture, then return to pot and cook until bubbling. Remove from heat and whisk in softened butter once cooled.
For the quark filling, mix soft butter and egg yolks, then add sugar, lemon zest, and juice. Sift in dry ingredients and mix. Fold in whipped cream and quark.
Roll out dough and line a springform pan. Pour in quark filling and spread pastry cream on top. Bake at 325°F for 55-60 minutes. Let cool in the oven for 2-3 hours, then cool completely and dust with powdered sugar if desired.
