For dough, add cut butter to flour in a large mixing bowl. Add egg yolk & sour cream, mixing into a ball. Doing this with your hand works well but an electric mixer also does the job. Wrap in floured wax paper & refrigerate overnight.
Mix the sugar, walnuts & cinnamon. Set aside.
Roll out one quarter of dough at a time to form a circle. Sprinkle dough evenly with sugar mixture & then cut into about 12 wedges (think of cutting a pizza into narrow slices). Roll from wide end inward.
Brush with egg white & sprinkle with brown sugar. Bake 15 minutes on a lightly greased cookie sheet.
Once cool sprinkle with confectioner’s sugar to finish.