Combine the flour, sugar, dry yeast, mahleb, mastic and salt in the bowl of the mixer.
Add the eggs and milk and knead with dough hook until all ingredients are well-incorporated.
Add each chunk of butter separately to allow the chunks to be absorbed in the dough, and continue kneading until the dough is soft, elastic and pulls away from the sides of the bowl.
Transfer dough into a container, cover with a towel and let it rest in a warm spot for 2 hours, until it becomes double in size.
Split the dough in three equal halves and roll each halve into a rope. Join the top of the ropes together and braid them to form a braided loaf.
Place the tsoureki on a non-stick cooking dish or tray and let it rest in a warm spot for 1 hour to rise.
Whisk the remaining egg with a little water using a pastry brush, and afterwards brush the whisked egg over the tsoureki.
Garnish with grated almonds and bake in pre-heated oven to 170oC for about 30 to 40 minutes.