Knead all the dough ingredients, except cornstarch, into a soft dough and divide into 12 equal balls.
Place onto a floured surface and let rest for an hour.
Roll out each ball into a circle the size of a large dinner plate and sprinkle cornstarch on top, through a sieve. Roll out the next ball in the same way and place on top of the first. Repeat until you have six dough circles, one on top of the other. You will have two large piles of six each.
Roll out the pile of six into one very large circle, sprinkling with cornstarch and stretching to make it as large as possible. Lightly smear the circle with the butter-oil mixture and roll up like a jelly roll. Repeat with the second pile of six.
You now have two large rolls. Cut each one in half so that you have four shorter rolls. Place rolls on a tray, separating each one with plastic so that they don’t stick to one another, and cover the entire tray with plastic wrap.
Dust your working surface with cornstarch and slice each roll into 3/4-inch (two centimeter) slices.
With your hands dusted in a little cornstarch, flatten each piece slightly and create a well in the middle. Fill with cheese and fold over to close.
Place on a tray lined with Gefen Parchment Paper and brush with beaten egg yolk.
Bake at 350 degrees Fahrenheit (180 degrees Celsius) until golden.