In a small bowl, combine the lukewarm water, instant yeast, and 1 teaspoon of sugar. Stir gently and let sit for 5 to 10 minutes until foamy. If it does not foam, your yeast may be old. Start again with fresh yeast before proceeding.
In a large bowl, whisk together the flour, sugar, mahleb, salt, and baking powder. Make a well in the center of the dry ingredients.
Into the well, add the foamy yeast mixture, eggs, milk, melted butter, and vanilla. Mix together until a dough forms. The dough will be sticky. This is correct. Do not add extra flour.
Cover the bowl tightly with plastic wrap and then a kitchen towel. Place in a warm spot. The top of the stove, inside an oven with just the light on, or near a sunny window all work well. Let rise until doubled in size, 1.5 to 2 hours.
Preheat your oven to 350 degrees F. Line two baking sheets with parchment paper.
Divide the dough into 12 to 15 equal pieces. Roll each piece into a rope about 10 to 12 inches long. Fold the rope in half and twist the two strands around each other, then press the ends together to seal. Place on the prepared baking sheets.
In a small bowl, beat together the egg and water for the egg wash. Brush each twist generously with egg wash.
Sprinkle nigella seeds or sesame seeds over each twist.
Bake for 20 to 25 minutes until deep golden brown. The color should be rich, not pale. Let cool on a wire rack before serving.
