Take the phyllo pastry sheets out, cut them along their length to obtain long strips, about 4 fingers wide.
Beat the yoghurt with the eggs, add the baking powder, feta cheese and flour. Stir and begin to smear each strip, while also putting small pieces of butter on top of them.
Fold the edges and begin to roll them up into triangles. Arrange them in an oven dish covered with oiled baking paper, with spaces in between them.
Smear the phyllo pastries with the egg yolk for a nice color and bake in a 338°F (170 °C) oven until reddened.