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 0 cm thick. Cut out rectangles about 2 X 8 cm. Spread with 1 t of filling. Roll up jelly roll fashion. Place on greased baking sheets and curve to form crecents. Bake at 180ºC . for 15 to 20 minutes, or until lightly browned. Cool on wire racks and store in airtight tins. Makes 4 to 5 dozen crecents.