1/2 cup unsalted butter
8ounces dark chocolate chips
1 teaspoon vanilla extract
5 egg yolks (room temperature)
1/3 cup granulated sugar
8 egg whites (room temperature)
2 tablespoons granulated sugar
1/4 teaspoon cream of tartar
Preheat oven to 375 degrees and adjust the rack to the lower-middle position. Generously butter a 2-quart souffle dish (or six 8-ounce ramekins), then sprinkle with granulated sugar, tapping and turning the bowl until the sugar has completely coated the butter. Place in the refrigerator until needed.
In a medium bowl melt butter and dark chocolate. Place the bowl over a small saucepan of simmering water, stirring occasionally until melted. Off heat, whisk in the vanilla extract and salt; set aside.
Using an electric mixer on medium speed, beat the egg yolks with ⅓ cup sugar until the mixture is thick and very pale yellow, 3 to 4 minutes. Use a rubber spatula to gently fold in the chocolate mixture; set aside.
Using an electric mixer on medium-low speed, beat the egg whites and cream of tartar until frothy. Increase the speed to medium and beat until soft peaks form. Increase the speed to medium-high and gradually add the remaining 2 tablespoons of granulated sugar; continue to beat until stiff, glossy peaks form.
Add one quarter of the egg whites to the chocolate mixture and stir it in to lighten up the mixture (the chocolate egg mixture will have thickened a little bit). Add the remaining egg whites all at once and use a rubber spatula to gently fold them in until no white streaks remain.
Transfer the chocolate mixture to the prepared souffle dish and smooth the top into an even layer. Trace a circle with your finger in the batter about ½-inch from the edge of the pan (this will help give it a nice even rise). Place the souffle dish on a rimmed baking sheet and place in the oven. Bake until it has fully risen, the outside is set, and the center just barely jiggles when the pan is moved, 30 to 40 minutes for large souffle and 20 to 25 minutes for 8-ounce ramekins. Remove from the oven, dust with powdered sugar, and serve immediately.