Make the batter:
Melt the 4 tablespoons of butter and the chopped chocolate in a double boiler or a heatproof bowl in the microwave (20-second intervals, stirring between). Let it cool for 3–5 minutes.
Whisk in egg yolks, vanilla extract, and salt into the chocolate mixture. Set aside.
In a clean mixing bowl, beat egg whites and cream of tartar on high speed until soft peaks form (~2 mins).
Gradually add the sugar, 1 tablespoon at a time, beating for 5 seconds between additions.
After all sugar is added, beat until stiff glossy peaks form (~2 more minutes). Do not over-beat.
Gently fold the egg whites into the chocolate mixture in 3 separate additions, folding completely between each addition.
Refrigerate the batter for 5–10 minutes while preheating the oven and preparing ramekins.
Adjust oven rack to lower third position and preheat to 400°F (204°C).
Prepare ramekins:
Brush four 6-ounce ramekins with softened butter. Sprinkle with granulated sugar, rotating to coat fully. Discard excess sugar.
Place ramekins on a baking sheet.
Spoon batter evenly into ramekins.
Smooth the tops and run a knife or thumb around the rim to create a “channel.”
(Optional: Wipe edges with a towel for appearance.)
Bake:
Place baking sheet in the oven, then reduce temp to 375°F (191°C).
Bake for 13–14 minutes, or until edges are set and center just jiggles when tapped.
Avoid opening the oven.
Serve immediately.
Soufflés will begin to fall within minutes. Still sponge-like under a crackly top.
Store leftovers covered in the fridge for up to 3 days.