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1
Preheat the oven to 350F.
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2
Halve the butternut squash lengthwise and remove the seeds and strings.
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3
Brush the insides with oil; season with salt and pepper.
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4
Turn the squash over and place the halves side by side on a rack in a roasting pan, cut side down.
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5
Bake for 45 minutes, until fork-tender.
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6
Remove the squash from the oven, scoop out the flesh, and put it in a food processor; you should have 2 cups.
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7
Add the orange zest, rosemary, cinnamon, and butter and season with salt and pepper.
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8
Pulse to combine.
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9
Scrape the butternut squash puree into a bowl and cool to room temperature.
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10
Whisk in the egg yolks to cream out the filling.
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11
In a separate clean bowl beat the egg whites and cream of tartar to stiff peaks.
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12
With a rubber spatula, fold one third of the beaten whites into the squash mixture to lighten it.
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13
Then gently fold in the rest.
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14
Grease a 2-quart souffle dish with softened butter, sprinkle with granulated sugar, and pour out any excess.
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15
(The butter and sugar will keep the souffle from sticking to the sides, allowing it to rise evenly.)
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16
Spoon the butternut squash mixture into the prepared baking dish and place on a cookie sheet.
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17
Bake on the middle oven rack for about 30 minutes.
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18
The souffle is done when it has puffed over the rim, the outside is golden, and the center jiggles slightly.
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19
Dust with confectioners sugar before serving.
