- Unsalted butter, room temperature, for baking dish
- 1/4 cup sugar, plus more for baking dish
- 8 ounces semisweet chocolate, finely chopped, or semisweet chocolate chips (1 cup)
- 1 teaspoon pure vanilla extract
- 3 large egg yolks, lightly beaten, plus 4 large egg whites
- 1/4 teaspoon cream of tartar
- Preheat oven to 350 degrees. Lightly butter a 1 1/2-quart tall-sided baking dish. Coat with sugar, tapping out excess. Set dish on a rimmed baking sheet.
- In a large heatproof bowl set over a pot of simmering water, combine chocolate, vanilla, and 1/4 cup water. Stir until chocolate is melted and mixture is smooth, about 10 minutes. Remove from heat and let cool to room temperature, 20 minutes.
- Stir egg yolks into cooled chocolate mixture until well combined. Set souffle base aside.
- In a large bowl, using an electric mixer, beat egg whites and cream of tartar on medium-high until soft peaks form, about 2 minutes. Gradually add sugar and beat until stiff, glossy peaks form, about 5 minutes (do not overbeat).
- In two additions, fold egg-white mixture into souffle base: With a rubber spatula, gently cut down through center and lift up some base from bottom of bowl. Turning bowl, steadily continue to cut down and lift up base until just combined.
- Transfer mixture to dish, taking care not to get batter on top edge of dish; smooth top. Bake souffle until puffed and set, 30 to 35 minutes. (Do not open oven during first 25 minutes of baking.) Serve immediately.
Serving Strategy: Dust the finished souffle with confectioners' sugar if you like, then serve straight away -- the souffle will begin to lose its lift and collapse as it cools.