Preheat the oven to 400 degrees. Position a rack so the souffle will sit in the middle of the oven. Spread the 1/2 tablespoon of butter evenly inside a 6-cup souffle dish (6 1/2-inches round x 3 1/2-inches high) with a paper towel. Add the 2 tablespoons of granulated sugar and tilt the dish to distribute the sugar evenly. Lightly tap out the excess and set aside.
Melt the 2 tablespoons of butter in a 9-inch round enamel pot, such as Le Creuset, over medium heat. Whisk in the flour and cook for 1 minute, whisking constantly. Whisk in the milk and 1/4 cup granulated sugar and cook for 1 minute, whisking constantly. Off the heat, cool for 1 minute and whisk in first the egg yolks and then the orange liqueur, vanilla, and orange zest and set aside.
Put the egg whites, cream of tartar, and salt in the bowl of an electric mixer fitted with the whisk attachment and beat on medium-high speed until they are doubled in volume and frothy. With the mixer running, slowly add the 1/4 cup of granulated sugar and beat until the egg whites form soft, very creamy (not firm!) peaks. Whisk 1/4 of the egg white mixture into the egg yolk mixture, then add all the egg whites and fold them carefully but thoroughly with a rubber spatula into the base.
Pour the mixture into the prepared souffle dish, not filling it more than 1 1/4 inches from the top of the dish (there may be extra mixture). Put your finger 1 inch down into the edge of the souffle mixture and run it all the way around, making sure you don't rub off the butter and sugar inside the dish. Place in the middle of the oven and immediately turn the temperature down to 375 degrees. Bake for 30 to 35 minutes, until the souffle is puffed and browned on top. Don't peek!!
Sift a light dusting of the confectioners' sugar on top and serve immediately.