Preheat oven to 400 degrees. Lightly butter 4 ramekins (small 7oz. baking cups). Coat with sugar, tapping out excess. Set on a rimmed baking sheet.
In a large heatproof bowl set over a pot of simmering water, combine chocolate, vanilla, and butter. 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 soufflé base aside.
In a large bowl, using an electric mixer, beat egg whites and cream of tartar (or lemon juice) 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, minutes. (Do not open oven during first 15minutes of baking.) Dust with powdered sugar if desired. Serve immediately
This recipe was provided by Kate V