Melt the butter in a small saucepan over low heat; when the solids separate, remove from heat.
Skim off the foam with a spoon, carefully keeping only the pure fat in another bowl.
In a bowl, dissolve the yeast and sugar in the water, add the flour, and once the ingredients are combined into a mixture, set aside for 20 minutes.
Preheat the oven for 5 minutes at 60°C and then turn it off to allow the dough to rise inside later.
In a mortar, crush the spices together with 1 tablespoon of sugar.
In the bowl of a stand mixer, add the eggs and sugar, use the paddle attachment, and mix for 2-3 minutes. Then add the yogurt, spices, and sourdough starter, and mix for 1 minute.
Begin adding the flour gradually.
When you have a dough, add the butter and mix until fully incorporated.
Transfer the dough to a well-oiled bowl, cover with plastic wrap, and place in the oven for 1 hour.
Then knead the dough; if making braided tsourekia, divide into smaller dough pieces of 120-150 g. If shaping round tsourekia, divide into four pieces.
Shape the tsourekia and place them on disposable baking trays, then place back in the oven or in a warm place and let them rise for 45 minutes until doubled in size.
Preheat the oven to 180°C. Mix the egg yolks with water and brush the tsourekia all over.
Bake the tsourekia for 30-35 minutes and let them cool completely on a rack.
