Soften butter.Mix with flour and add sour cream and egg yolk.Work into a ball.Then divide the ball into 4 smaller balls. Refrigerate for 4 hours or more.Then roll one ball out like a pizza, 1/8-inch thick.Sprinkle cinnamon, sugar and chopped nuts over the dough.Cut into 16 pieces like a crescent roll and roll up.Bake at 35~ for 25 minutes.