Preheat oven to 375 degrees F and adjust the rack to the lower-middle position. Generously butter a 2-quart souffle dish (or eight 8-ounce ramekins) and sprinkle with granulated sugar, then place in the refrigerator until needed.

In a medium heatproof bowl, melt together the butter and dark chocolate. 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 of the sugar until thick and very pale yellow, about 3 to 4 minutes. Gently fold in the chocolate mixture; set aside.

In a separate bowl, beat the egg whites and cream of tartar until frothy. Gradually add the remaining 2 tablespoons of sugar and continue to beat until stiff peaks form.

Add one quarter of the egg whites to the chocolate mixture and stir to lighten up the mixture. Gently fold in the remaining egg whites until no white streaks remain.

Transfer the chocolate mixture to the prepared souffle dish and smooth the top. Trace a circle about ½-inch from the edge of the pan with your finger.

Place the souffle dish on a rimmed baking sheet and bake until fully risen, the outside is set, and the center just barely jiggles, about 30 to 40 minutes (20 to 25 minutes for 8-ounce ramekins).

Remove from the oven, dust with powdered sugar, and serve immediately.