Preheat the oven to 425º. Move an oven rack to the lower third of the oven.
Coat the inside of an 8-cup souffle dish with 2 tablespoons of butter. Add 2 tablespoons of granulated sugar and roll it around the inside of the dish to coat. Discard the remaining sugar and set the dish aside.
In a small saucepan, heat the milk, 1/2 cup granulated sugar, and the orange zest until the mixture steams. Remove from heat and set aside.
While the milk is warming up, heat a 2-quart saucepan over medium heat and add the remaining 2 tablespoons butter. Let the butter melt, then add the flour and salt and cook, stirring, until the mixture is foamy, about 2 minutes. With the saucepan still over the heat, whisk the flour mixture and add the hot milk all at once. Whisk until the mixture becomes thick and velvety, about 2 minutes. Remove from the heat and transfer to a large bowl. Let the mixture cool for 5 minutes. This is the base for your souffle.
Whisk the egg yolks, one at a time, into the souffle base. Add the Grand Marnier and set aside.
Using a mixer set on medium speed, beat the 7 egg whites until they are foamy. Turn the speed to high. When the whites form soft peaks, add the remaining 1/4 cup granulated sugar slowly. Continue to beat on high speed for 1 minute. The whites should become glossy and firm.
To assemble the souffle, add one-fourth of the egg whites to the souffle base. using a large flat spatula, gently turn and fold the souffle base up and over the top of the egg whites to lighten the mixture. Add half the remaining egg whites and fold, leaving some streaks of white. Then add the remaining egg whites and gently fold them in completely. Be careful not to overmix or the whites will deflate, leaving you with an under-risen souffle. The secret to success here is in folding until each addition of egg whites is almost incorporated, then adding the next batch and so on until all the whites are incorporated.
Gently transfer the souffle batter into the prepared dish and flatten the top with a spatula. Run a finger or spoon around the edge of the souffle, leaving a trough about 1/2-inch deep. This will allow the souffle to rise even higher.
Put the souffle in the oven and immediately turn the heat down to 375º. Bake for 20 minutes, then pull out the oven rack, and carefully dust the souffle with the confectioner's sugar. Gently push the rack back into the oven and close the door. (Don't over worry about a stray noise or bump affecting your souffle. They are much hardier than you think.) Check the souflle after 15 minutes and gently jiggle it. If it moves slightly in the center the souffle is done. If it is loose on the edges, bake for another 5 minutes. It should be brown and crusty on top.
Remove the souffle from the oven and serve immediately.