Mix yeast, flour, butter, sour cream and egg yolks thoroughly until dough is formed. Cover and let dough rest 1 hour. For filling beat egg whites until soft peaks form. Fold in nuts, sugar, and vanilla. Roll dough 1/8-inch thick. Cut out rectangles about 2 X 3-inches. Spread with 1 t of filling. Roll up jelly roll fashion. Place on greased baking sheets and curve to form crecents. Bake at 350 degrees F. for 15 to 20 minutes, or until lightly browned. Cool on wire racks and store in airtight tins. Makes 4 to 5 dozen crecents.