How to Make It
Preheat the oven to 300°. Butter a 9-by-13-inch glass baking dish. In a clean bowl, beat the egg whites with the cream of tartar until the whites begin to stiffen. Add 1/4 cup plus 2 tablespoons of the confectioners' sugar and continue beating until the whites hold stiff peaks and become glossy.
In a separate bowl, whisk the egg yolks with the remaining 1/4 cup of confectioners' sugar until light and foamy, about 3 minutes. Stir in the Grand Marnier and set aside.
Gently spread the egg whites in the prepared baking dish. Pour the yolk mixture over the whites, leaving a 1-inch border all around. Bake in the oven for 7 to 10 minutes, or until the whites and yolks are just set. Serve at once.