Dissolve the yeast in hot water along with some sugar and flour. Wait for about 10 minutes to allow the yeast to activate. Because of the addition of sugar, the mixture should get frothy and release carbon dioxide. Make sure your bowl isn't too small or it may overflow.
Meanwhile, melt the butter and warm up the milk to a simmer in separate saucepans.
In a mixing bowl, beat the eggs with the remaining 1 cup of sugar and vanilla extract. Then slowly add the hot milk, beating constantly, followed by the melted butter until well combined. Then beat in the yeast mixture. The liquid mixture should be fairly warm.
In another large bowl, combine the flour, mahleb, baking powder, and salt. Create a well in the center of the dry mixture and pour in the liquid mixture.
Use a large spoon to gently mix the dry mixture into the liquid, starting from the center and moving outward until all the dry mixture is moistened. Start using your hands to finish mixing and knead the dough.
Transfer the dough to a floured surface and knead for about 10 to 15 minutes.
Cover with foil and a towel and let it rise for about 2 hours. Knead again once it has doubled in size, for at least 5 minutes, and let it rise for one more hour.
Divide the dough into 4 equal pieces and divide each of those into three equal parts.
Roll the three parts of each to form a thick stick and braid them.
Brush the top surface with egg yolk and sprinkle with sesame seeds.
Preheat the oven to 356°F and bake for about 10 minutes. Then reduce the temperature to 320°F and bake for another 20 minutes.
