Beat the egg whites 30 seconds until foamy, add cream of tartar and pinch of salt. Beat until soft peaks are formed. Sprinkle in the 1 tablespoon sugar and beat until stiff peaks are formed. Do not overbeat. Set aside.
Beat the orange liqueur, vanilla and orange zest into the sauce. Stir in a fourth of the beaten egg whites. Delicately fold in the rest of the egg whites.
Turn the soufflé mixture into the prepared molds leaving a generous space between the top of the soufflé and the rim of the mold. If the mold is too full, the soufflé will spill over as it rises.
Place the molds on a baking sheet (if desired), place on the second-lowest rack of the oven, and immediately turn the oven down to 375 degrees.
For minis, bake 8 to 9 minutes or until tops are puffed and golden (or test with a straw: it should come out clean when plunged into the soufflé through the side of the puff). Remove from oven and sift powdered sugar on top. Serve immediately.
For the 6-cup mold, bake for 20 minutes, then (when the soufflé has begun to puff and brown) open oven and quickly sprinkle the top with powdered sugar. After a total of 30 to 35 minutes of baking, the top of the soufflé should be nicely browned, and a straw plunged into the soufflé through the side of the puff should come out clean.