Rinse, halve and stone the peaches. Puree 2 of the peach halves in a food processor. Place the puree in a bowl and add the sugar, crushed macaroons and the egg yolk. Cover each peach half with this filling and place the peach halves in a buttered baking dish. Sprinkle the remaining butter over the peaches and bake at 350 degrees for about 45 minutes. Serve hot or cold, with or without heavy cream.