Proof the yeast. Add in a bowl the lukewarm water, a pinch of sugar and yeast and stir. Wrap well with plastic wrap and set aside until the water-yeast mixture becomes frothy and foamy.
Mix the wet ingredients. In a saucepan add the butter, sugar and milk. Place over very low heat and stir the mixture, until the butter has melted and the sugar has dissolved. Pour the butter mixture in a large bowl and whisk in the eggs. Add the yeast mixture and whisk well to combine.
Mix the dry ingredients. Add to the bowl of a stand mixer flour, ground mastic and mahlepi, orange zest and the butter-egg-yeast mixture. Using the dough hook, run the mixer first at low speed, until the ingredients start to combine and then mix at medium-high speed for about 15 minutes, until the dough comes together. The dough should be sticky and soft.
Let the dough rise. Cover the dough with plastic wrap and place in a warm area, until it doubles in size (for about 1-2). If your kitchen is cold, then preheat the oven at 30C, turn it off and place the bowl inside.
Knead and let it rise again. Gently deflate the risen dough with your hands and transfer it to a lightly oiled container. Cover the dough with a think later of oil, cover it and let it rest in the fridge overnight.
Shape the Mangauna. With a regular knife or a dough knife, cut the dough mass into 6 equal portions (three for each Greek Easter bread). Resist the urge to flour your counter top. Instead wipe it with some oil to avoid Without flouring the surface, roll out each piece of dough it a little bit with your hands. Stretch the dough from its ends, this will help giving the bread its stringy texture. Braid the bread, place it on a baking sheet lined with parchment paper. Cover the baking sheet and let it rise again for 20 minutes.
To make the egg wash, Whisk together in a small bowl an egg and 1 tbsp water. Brush the top of each Greek Easter bread with the egg.
To garnish the tsoureki, sprinkle almond silvers or sesame seeds and bake in preheated oven at 170C for about 40-50 minutes, until nicely browned and fluffy.
To make the syrup, In a deep small pot add the sugar and water and bring to the boil. As soon as the sugar dissolves, remove the pot from the stove and ladle the hot syrup over the hot tsoureki.
Serve immediately the Tsoureki while it is still hot, or serve it later covered with a towel. Tsoureki is an excellent freezer food. You can wrapped it tightly with stretch tight and freeze it for several weeks.