Melt remaining butter. Brush the bottom of a 13x9 baking dish with melted butter. Place 10 sheets of filo dough one sheet at a time into the baking dish, brushing each sheet with melted butter. Put filling into baking dish. Add 10 more sheets of filo dough, buttering each one. Beat the egg yolk and brush the top sheet of filo with the egg. Cut into squares before baking (12 or so).
Bake at 350 degrees for about 1 hour or until golden brown.
Notes
*If the package has 20 13 x 20-inch sheets, just cut the stack in half to make 20 13 x 10-inch sheets and save the other stack for another time.