Prepare the leaven. In a bowl add ½ cup milk, ½ cup flour, 2 tbsps sugar and yeast and make a porridge. Cover and let it bubble and rise until we prepare the rest of the remaining ingredients. In a large bowl (large enough to have space for the dough to rise) add the flour (make sure that it is the right to use for tsoureki), machleb, cardamom and mastic and mix well.
Heat milk, sugar and half of the butter in a saucepan. If we have a thermometer, the milk should not exceed 40C. Remove the pan from the heat and break the eggs in. Add the zest and mix well until the mixture is homogeneous.
In the bowl with the flour, add the milk mixture and the yeast and start kneading. At first it will stick a lot, so add flour. Sprinkle little by little from the half cup you have and continue kneading for a little longer. It will take you 5 to 7 minutes to knead. When it becomes a soft ball, add the remaining warm butter and start folding. Be careful not to knead at this point. Add a little butter and pull the dough to fold. Don’t wait for the dough to absorb all the butter to add the next amount, as it will absorb it during rising.
Cover the bowl well and leave it in a warm place to triple in volume. It will take about 2 - 2 ½ hours depending on the room temperature. If you want, you can knead it late at night and leave it in a cool place all night and shape it in the morning.
Once it is ready, deflate it and knead for another minute. Then turn the dough over and divide it into four equal parts or the size you want. Separate each piece of dough in 2 if you are going to make the braid with the 4 laces or in 3 for the classic braid. Sprinkle very lightly with flour and shape into strings of equal thickness to make the braid. Put the buns in a pan with parchment paper and let them rise again in a warm place until they double in volume. It will take about 30 minutes. Meanwhile, preheat the oven to 200 C.
Spread the surface of the tsoureki with egg, let it dry for 1 minute and spread again. Sprinkle with almond flakes or sesame seeds and bake for about 20 minutes. Remove the tsourekia from the oven and let them cool. To keep them fluffy, wrap them in cling film.