1. For the strudel: Preheat oven to 400°. Melt 4 tbsp. of the butter in a small skillet over medium-low heat and cook until lightly browned. Pour clarified butter into a small bowl, discarding milk solids. Beat confectioners' sugar and 7 tbsp. of the butter together in a mixing bowl with an electric mixer on medium speed until light and fluffy. Add granulated sugar, vanilla, lemon zest, and salt and beat for 1 minute. Add yolks and beat for 1 minute more. Beat in cheese and sour cream, then beat in flour. In a clean bowl with clean beaters, beat egg whites until soft peaks form. Fold whites into cheese mixture and set aside.