1. In 2-quart saucepan over low heat, melt butter or margarine. Stir in flour and salt until blended; gradually stir in milk; cook, stirring constantly, until mixture thickens and boils; continue to cook 1 minute. Remove saucepan from heat.
2. With wire whisk, beat 3 tablespoons sugar into milk mixture. Rapidly beat in egg yolks all at once until well mixed. Refrigerate egg-yolk mixture to cool to lukewarm, stirring mixture occasionally. Stir in orange liqueur and grated orange peel.
3. Preheat oven to 375°F. Grease 2-quart souffle dish with butter or margarine and lightly sprinkle with sugar.
4. In large bowl, with mixer at high speed, beat egg whites until stiff peaks form. With rubber spatula or wire whisk, gently fold egg-yolk mixture, one-third at a time, into beaten egg whites.
5. Pour mixture into souffle dish. With back of spoon, about 1 inch from edge of dish, make 1-inch-deep indentation all around in souffle mixture (the center will rise higher than the edge, making a top-hat effect when the souffle is done). Bake 30 to 35 minutes, until knife inserted under top hat comes out clean.
6. Meanwhile, if using whipped cream, beat heavy or whipping cream until soft peaks form; cover and refrigerate.
7. When souffle is done, serve immediately. Pass whipped cream in bowl to spoon onto each serving, if you like.