PARIS Pastry chef Arnaud Delmontel rolls out dough for croissants and pain au chocolat that later emerge golden and fragrant from the oven in his Paris patisserie. The price for the butter so essential to the pastries has shot up in recent months, by 25 percent since September alone, Delmontel says. But he is refusing to follow some competitors who have started making their croissants with margarine. “It’s a distortion of what a croissant is,” Delmontel said. "A croissant is made with butter .”