Mix the feta cheese and parsley in a bowl, then set aside.
Mix the plain yogurt, egg, milk, and sunflower oil in a large bowl until you get a smooth batter.
Grease the baking tray with oil. Then place one of the yufka into the tray, don't cut off the excess parts; leave them outside the tray to be folded in later. Lay the other yufka sheets randomly by cutting them with your hands. Don't try to lay it straight; being wavy will allow it to cook better. Drizzle the batter you made on each layer. Repeat the process until half of the yufkas are layed.
Sprinkle the cheese mixture on the mid-layer. Then cover the cheese layer with cut yufka pieces and drizzle the batter. Repeat the process until you use all of the yufkas. Fold the outer parts of the first yufka sheet on top. Drizzle with batter after folding each side of the yufka. Pour the remaining batter and brush the egg yolk on top. Sprinkle the sesame and black sesame seeds on top.
Bake the borek in a preheated oven at 180°C/355°F for about 35-40 minutes until golden brown.