Step 1: Prepare the Ramekins
Position an oven rack in the lower third of your oven and preheat to 375°F (190°C). Using a pastry brush, generously coat the inside of six 6-ounce ramekins with softened butter, brushing upward in vertical strokes from the bottom to the rim. This upward motion is crucial as it helps the soufflé climb the sides. Pay special attention to the rim and corners. Place cocoa powder in a small sieve and dust the inside of each buttered ramekin, rotating to coat evenly. Tap out excess cocoa. Place prepared ramekins on a large baking sheet and set aside.

Step 2: Melt the Chocolate
Fill a medium saucepan with 1-2 inches of water and bring to a simmer over medium heat. Place chopped chocolate and 3 tablespoons butter in a heatproof bowl that fits snugly over the saucepan without touching the water. Place bowl over the simmering water and stir occasionally until chocolate and butter are completely melted and smooth, about 3-4 minutes. Remove bowl from heat and stir in heavy cream until fully combined and glossy. Set aside to cool slightly, about 5 minutes. The mixture should be warm but not hot when you add the egg yolks.

Step 3: Add Egg Yolks
Whisk egg yolks briefly in a small bowl. Add 2 tablespoons of the granulated sugar to the yolks and whisk until slightly thickened and pale yellow, about 1 minute. Stir vanilla extract into the chocolate mixture, then add the egg yolk mixture. Whisk vigorously until completely smooth and well combined. The mixture will be thick and glossy. Set aside at room temperature while you whip the egg whites.

Step 4: Whip the Egg Whites
In a large, completely clean and grease-free mixing bowl, combine egg whites, salt, and cream of tartar (if using). Using an electric mixer, beat on medium speed until the whites become foamy and opaque, about 1 minute. With the mixer running, gradually add the remaining sugar (6 tablespoons) one tablespoon at a time, allowing about 10 seconds between additions. Once all sugar is added, increase speed to medium-high and continue beating until stiff, glossy peaks form, about 3-4 minutes total. The peaks should stand straight up when you lift the beaters, and the meringue should look smooth and shiny, not grainy or dry.

Step 5: Fold the Mixtures Together
Add about one-third of the whipped egg whites to the chocolate mixture. Using a rubber spatula, stir vigorously to lighten the chocolate base – you can be rough with this first addition. Add half of the remaining egg whites and fold gently using a cutting and turning motion: Cut down through the center of the mixture, sweep across the bottom, then bring the spatula up the side and fold over the top. Rotate the bowl a quarter turn and repeat. Continue folding just until you see a few white streaks remaining, about 15-20 strokes. Add the final portion of egg whites and fold just until no white streaks remain and the mixture is uniform in color, about 10-15 more gentle strokes. The batter should be fluffy, airy, and voluminous.

Step 6: Fill the Ramekins
Divide the soufflé mixture evenly among the prepared ramekins, filling each to about ¼ inch below the rim. Gently tap the baking sheet on the counter once or twice to release any large air bubbles. Using your thumb, run it around the inside rim of each ramekin, creating a shallow groove between the soufflé mixture and the edge – this helps the soufflé rise straight up rather than mushrooming over the sides.

Step 7: Bake the Soufflés
Immediately place the baking sheet in the preheated oven on the lower rack. Bake without opening the oven door for 12-15 minutes. The soufflés are done when they’ve risen about 2 inches above the rim, the tops are set and slightly cracked, and they jiggle just slightly when gently shaken. For a more molten center, bake for 12 minutes. For a more set center, bake for 15 minutes.

Step 8: Serve Immediately
Remove the baking sheet from the oven and dust the tops of the soufflés lightly with powdered sugar using a fine mesh sieve. Serve immediately – place each ramekin on a small plate with a dessert spoon. Instruct guests to crack the top with their spoon and enjoy the molten center. If desired, serve with a dollop of whipped cream, a scoop of vanilla ice cream, or fresh berries on the side.