Preheat oven to 350 degrees, cover souffle dishes with butter and sugar, tap out excess sugar. Set dishes on baking sheet.
Set a heatproof bowl over simmering water. Combine chocolate, vanilla, and 1/4 cup water. Stir until chocolate is melted(about 10 min). Set aside(20 min).
Add egg yolks to chocolate mixture.
Using an electric mixer, beat egg whites and cream of tartar until soft peaks form. Then add sugar, a little at a time, and beat until stiff peaks form.
Fold egg white mixture into chocolate mixture. Use a spatula.
Transfer mixture into 6 dishes, evenly. Bake until puffed, around 30 minutes.