In a bowl add the lukewarm water, active dry yeast and 1/4 tsp of the sugar. Whisk well, cover with a towel or plastic wrap and let it rest in a warm place for 15 minutes.
In the meantime, in a small saucepan pour the milk, 1/2 cup of cubed butter and the remaining sugar. Add the ground mastic, ground mahleb, orange zest, vanilla extract and salt and heat over medium-low heat.
Stir, just until the butter is melted. Immediately, remove from heat and allow the mixture to come at room temperature.
While the butter mixture is cooling down, in another bowl whisk together 4 of the eggs and the orange juice.
Incorporate the room temperature butter mixture into the bowl with the eggs.
Uncover the yeast mixture. By now, the yeast should be activated and the surface will be bubbly.
Pour the wet mixture into the yeast mixture and whisk for a minute.
Melt the remaining 1/4 cup of butter and set aside.
Add the sifted bread flour in the yeast mixture and mix using a wooden spoon or by hand. See notes, if using a mixer.
When the flour is incorporated, knead the dough for about 15 minutes while gradually adding the melted butter, until it is no longer sticky. The dough will be soft and stretchy.
Grease a large bowl with one teaspoon of oil and place the dough in. Cover with a towel or plastic wrap and let the dough rest and double in size in a warm place. It will take about 2 hours.
Line two baking sheets with parchment paper and set aside.
Once the dough has doubled in size, gently deflate it with your fingers and cut into 6 equal pieces.
On a clean surface, take one piece of the dough and roll it with your hands. Hold the edges of the dough and stretch it by shaking it up and down until you have a long string.
Repeat the same procedure with the other two pieces of dough.
Place the three long strings of dough next to each other and connect the top edges using your fingers. Braid the dough and connect the bottom edges. See notes for braiding instructions.
Place the braided bread in the lined baking sheet.
For the Nutella filled tsoureki, roll the dough into three long strings as above. Using a rolling pin, roll each string flat and spread with Nutella.
Pinch the edges of the string together, to seal the filling and braid the same way as before.
Cover the breads with a towel or plastic wrap and let them rise once again for 30 minutes in a warm place before baking.
Preheat oven to 350°F (180C).
Mix one egg with 2 tablespoons of water and brush the top of each bread.
Sprinkle with almond slivers or sesame seeds.
Bake for 35 to 40 minutes changing the baking sheets halfway cooking, until the breads are golden brown on top.
Remove from oven and let them cool for 15 minutes in the baking sheets before transferring to cooling racks. See notes for topping tsoureki with red dyed eggs.
Enjoy!