1. Put the sugar, the ground mahlepi and mastic crystals into the mixing bowl and set 30 seconds / speed 10.
2. Add the eggs, butter, milk and yeast and warm 3 minutes / 37 ° C / speed 6.
3. Add the flour into the mixing bowl and set 10 seconds / speed 7. Then knead 3 minutes / kneading speed. Empty the dough in a bowl, cover with a clean kitchen towel and leave in a warm place for about 1/2 to 1 hour until it rise.
4. Divide dough into three equal peaces and roll each peace into a rope. Join the three ropes at one end, plait, and join again at the other end.
5. Place the tsoureki on a baking paper and cover it with another sheet of baking paper. Let rise again in a warm place for about 1/2 hour. Brush with whisked egg, and bake in a medium oven (160-170 ° C) for 20-30 minutes or until dark golden brown.