Soak the yeast in 1 glass of warm water, add 1 teaspoon of molasses, mix and leave for 10 minutes. Put 3 cups of flour in a mixing bowl, make a well in the middle and add the leavened water, egg white and salt. Knead until you acquired a soft dough. Cover it with a clean cloth, let it rest for half an hour in a warm place. Melt the butter, add the tahini and mix well. Divide the dough into walnut-sized balls. Open the meringues in 20 cm diameter and spread the tahini mixture. Carefully fold it into a long roll. Twist the two ends of the dough with your hands in the opposite direction to form a twist. After you stretch it as far as you can, wrap it like a rose. Give it a round shape by pressing lightly on it. Place on greased tray. Let rise for half an hour at room temperature. Brush egg yolk on the dough. Sprinkle with chopped walnuts, if desired. Bake in the oven at 180 degrees for 20-25 minutes and serve hot or cold.