In a small pan, melt margarine with yogurt until smooth. Little by little add in the flour, and knead until you have a soft ball of dough. For the filling: To make béchamel sauce melt the margarine over a low heat, then add in the flour a bit at a time, stirring constantly with a wooden spoon so that the mixture does not get lumpy . Pour in the milk gradually by stirring constantly. Remove pan from the heat and season with salt and pepper. Add in the grated gruyere cheese, feta cheese, chicken (cut into small pieces), eggs, basil and mix until combined. Divide the dough in small round balls (about the size of your fist), then press them into small thick round discs. Place a heaping spoonful of filling on each disc, then fold dough over to form half-moon shaped pies and pinch the edges of the dough together to prevent filling from spilling out. Brush each pie with milk and sprinkle with sesame seeds. Grease a baking pan with butter, arrange chicken pies by leaving space between them, and bake in a moderate oven for about 20 min.