Make the dough: In the bowl of a stand mixer fitted with the paddle attachment, cream the butter with the sour cream and the egg yolk until light and fluffy. Add 2 cups of the flour and salt and mix until a smooth dough forms. Wrap in plastic and refrigerate for 2 hours. In a small bowl, mix the remaining 1/2 cup flour with the confectioners’ sugar.
Make the filling: In a medium bowl, toss all the filling ingredients to combine.
Preheat the oven to 375° F. Divide the dough into 3 equal parts. Working with one piece at a time, roll a piece of dough using the flour-sugar mixture into a 12-inch circle, 1/8-inch thick. Sprinkle 1/3 of the filling mixture over the circle and lightly press in. Cut the circle of dough into 12 wedges.
Starting at the widest part, roll up each wedge of dough and shape into a crescent. Place on a sheet pan, 1-inch apart. Repeat with the remaining dough and filling until all 3 dozen rugelach are made.
In a small bowl, beat the egg white until frothy and use to brush all the cookies. Bake, rotating trays halfway through, until golden brown, 22 to 25 minutes. Let cool completely, then serve.