Preheat oven to 400°F.
Generously butter shallow round baking dish or gratin pan.
Beat egg yolks with 1 tablespoon liqueur and orange peel in medium bowl.
Gradually add 1/4 cup sugar, beating constantly until yolks are thick and lemon colored and mixture forms a ribbon when beaters are lifted.
Beat egg whites with salt and cream of tartar in large bowl until soft peaks form.
Add remaining 1/4 cup sugar 1 tablespoon at a time and continue beating until whites are stiff and glossy.
Gently stir 1/4 of whites into yolks, then fold yolk mixture back into whites.
Spoon mixture evenly into prepared dish (or pipe from pastry bag).
Sprinkle with powdered sugar.
Bake until souffle is puffed and browned, about 12 minutes.
Remove from oven, ignite heated liqueur and pour over souffle.
Spoon omelet onto heated serving plates.