Make the Batter - Melt butter and chocolate in a double boiler or microwave (20-second intervals). Let cool 3-5 minutes.
Add Yolks - Whisk in egg yolks, vanilla extract, and salt. Set aside.
Whip Egg Whites - Beat egg whites and cream of tartar on high until soft peaks form (~2 mins). Gradually add sugar, beat until stiff glossy peaks (~2 more minutes). Do not over-beat.
Fold - Gently fold egg whites into chocolate mixture in 3 separate additions.
Chill - Refrigerate batter 5-10 minutes while prepping ramekins.
Prep Ramekins - Adjust oven rack to lower third. Preheat to 400°F. Brush four 6-oz ramekins with butter, coat with sugar. Place on baking sheet.
Fill - Spoon batter into ramekins. Smooth tops and run a knife around the rim.
Bake - Place in oven, reduce temp to 375°F. Bake 13-14 minutes until edges are set and center just jiggles.
Serve Immediately - Souffles will begin to fall within minutes!
Pairs well with: Powdered sugar, whipped cream, and dramatic presentation.