Preheat the oven to 350Â°F. Halve the butternut squash lengthwise and remove the seeds and strings. Brush the insides with oil; season with salt and pepper. Turn the squash over and place the halves side by side on a rack in a roasting pan, cut side down. Bake for 45 minutes, until fork-tender.
Remove the squash from the oven, scoop out the flesh, and put it in a food processor; you should have 2 cups. Add the orange zest, rosemary, cinnamon, and butter and season with salt and pepper. Pulse to combine. Scrape the butternut squash puree into a bowl and cool to room temperature. Whisk in the egg yolks to cream out the filling.
In a separate clean bowl beat the egg whites and cream of tartar to stiff peaks. With a rubber spatula, fold one third of the beaten whites into the squash mixture to lighten it. Then gently fold in the rest.
Grease a 2-quart soufflĂ© dish with softened butter, sprinkle with granulated sugar, and pour out any excess. (The butter and sugar will keep the soufflĂ© from sticking to the sides, allowing it to rise evenly.) Spoon the butternut squash mixture into the prepared baking dish and place on a cookie sheet. Bake on the middle oven rack for about 30 minutes. The soufflĂ© is done when it has puffed over the rim, the outside is golden, and the center jiggles slightly. Dust with confectionersâ€™ sugar before serving.
Edited: October 15th, 2009