Make the dough: In the bowl of a stand mixer fitted with the paddle attachment, cream the butter and the cream cheese until light and fluffy. Add 3 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°. Divide the dough into 6 equal parts. Working with one piece at a time, roll a piece of dough using the flour-sugar mixture into an 18-by-5 inch strip, 1/8-inch thick. Sprinkle 1/6 of the filling mixture over the strip and lightly press in. Starting at the wider length, roll up to seal in the chocolate.
Cut each rolled up strip into 12 pieces. Place on a sheet pan, 2-inch apart. Repeat with the remaining dough and filling until all 6 dozen rugelach are made.
Brush all the cookies with the egg white to coat. Bake until golden brown, rotating halfway through, 18 to 20 minutes. Let cool completely, garnish with more confectioners’ sugar, then serve.