1. Adjust oven rack to middle position and heat oven to 375°F. Using stand mixer, whip egg whites and cream of tartar together on medium-low speed until foamy, about 1 minute. Slowly add 1/3 cup sugar and salt, then increase speed to medium-high and continue to whip until stiff peaks form, 3 to 5 minutes. Gently transfer whites to clean bowl and set aside.

2. Using stand mixer (no need to wash mixing bowl), whip yolks and remaining 1/3 cup sugar together on medium-high speed until pale and thick, about 1 minute. Whip in orange juice, zest, and flour until incorporated, about 30 seconds.

3. Whisk 1/4 of whipped egg whites into yolk mixture until almost no white streaks remain. Gently fold in remaining egg whites until just incorporated. Gently fold grated chocolate into batter.

4. Melt butter in 10-inch ovenproof skillet over medium-low heat. Swirl pan to coat evenly with melted butter, then gently scrape soufflé batter into skillet and cook until edges begin to set and bubble slightly, about 2 minutes.

5. Transfer skillet to oven and bake soufflé until puffed, center jiggles slightly when shaken, and surface is golden, 7 to 11 minutes. Using potholder (skillet handle will be hot), remove skillet from oven. Dust soufflé with confectioners sugar and serve immediately.

Serves 6