In a large bowl, combine flour, powdered milk, cornstarch, salt, and baking powder.
Add the vegetable oil and water. Knead until the dough comes together and feels soft—not sticky or dry. Add a little more water if needed.
Cover and let the dough rest for 30 minutes.
In a separate bowl, mix the grated cheese, parsley, flour, and nigella seeds.
Roll out the dough and cut into 9 cm circles using a cookie cutter or glass.
Place about 1 tablespoon of filling in the center of each circle. Leave the edges clear to help with sealing.
Fold the dough over into a crescent shape. Braid the edges or press with a fork to seal tightly.
Heat the oil in a pan and fry sambousek in batches until golden on both sides.
Drain on paper towels and serve hot as part of a mezze spread or on their own.