Divide up the sheets into 2 piles. Sprinkle butter on 1 sheet, place the 2nd on top, distribute a little butter again and feta/egg mixture along the narrow end of the sheet, fold the 2 ends and wrap it up into a tight roll.
Arrange them in a tray with baking paper. Bake at 338°F (170°С) until reddened.
Smear them with cold water using a brush and cover for a few min. with a towel.