In a medium-sized bowl, whisk together the flour and salt. Cut in the butter until the mixture resembles coarse crumbs. Stir in the sour cream to create a soft dough. Wrap well and refrigerate overnight.
Remove the kringle dough from the refrigerator and divide in half. Wrap one piece and put back in refrigerator. Take a 24 inch long piece of parchment paper and lightly flour the surface. Put the dough on the paper and lightly sprinkle it and your hands with flour. Roll the dough into a long strip 5x26" (position diagonally on the parchment). Pick up the dough from time to time and sprinkle more flour underneath as needed to keep it from sticking.
To prepare the filling, mix all ingredients together. Cover and refrigerate until ready to use. Bring filling back to room temperature before using, or it will tear the dough.
With a dry pastry brush, carefully sweep away any excess flour off the surface of the dough. Place the filling down the center of the dough, in a strip about one inch wide. Gently take the edge closest to you and fold it up and over the filling. Brush the edge of the dough farthest away from you with egg white. Next, fold that edge to overlap the first, making a slightly flat, filled tube of dough. Brush off any excess flour, then carefully shape the dough into a circle. Seal the open ends together and brush the kringle with lightly beaten egg white. Sprinkle the top with the mixture of sugar and chopped nuts. Trim off the excess parchment and slide the kringle and parchment onto a baking sheet. Bake for 25 minutes at 375 degrees or until evenly golden brown. Remove from oven and cool completely on a wire rack. Dust with powered sugar before serving, if desired. Repeat with remaining dough.