Preheat oven to 375°F. Mix flour with butter until the mixture forms crumbs. Add salt and 3tablespoons sugar.
Dissolve yeast in lukewarm milk. Add sour cream, egg yolks and sherry. Knead together with the buttered flour and make a real hard dough out of it.
Line a 3-foot-square white cotton cloth with wax paper. Place dough in the middle. Bring the corners of the cloth together and tie very loosely, thus allowing ample space for the dough to expand. However, the bag must be sealed so no water can enter.
Take a very large dish, half fill it with warm (not hot) water, and place dough package in it. When dough rises to the surface and floats like a balloon, it has risen enough.
Take out the dough and gently untie it. Place it on a floured board and roll it out to ½-inch thickness. Cut into pieces 3 by 5inches.
Mix walnuts, hazelnuts, remaining 4tablespoons sugar, the light cream and plum jam.
Fill the centers of the pieces with the filling. Roll up each piece and twist into crescent shape.
Beat the whole egg with 1teaspoon water. Spread tops of the crescents with the egg wash. Place them on a buttered baking sheet, and bake in the preheated oven for 15 minutes.