Creating a chocolate soufflé requires precision and care, but by following these step-by-step instructions, you’ll be rewarded with a dessert that’s both impressive and delectable.

Preparing the Ramekins

1. Preheat the oven to 375°F (190°C). Place a baking sheet in the oven to heat up as well—this will help the soufflés rise evenly.
2. Grease the ramekins: Use a pastry brush to coat the insides of six 6-ounce ramekins with softened butter. Be thorough to prevent sticking.
3. Dust with sugar: Add a spoonful of granulated sugar to each ramekin and rotate them to ensure the sugar coats the buttered surfaces evenly. Tap out any excess sugar.

Making the Chocolate Base

4. Melt the chocolate and butter: In a heatproof bowl set over a pot of simmering water (double boiler method), melt the chopped dark chocolate and butter together. Stir until smooth, then remove from heat and let it cool slightly.
5. Prepare the egg yolk mixture: In a separate bowl, whisk together the egg yolks, vanilla extract, and salt. Gradually add the melted chocolate mixture, whisking continuously until well combined.

Preparing the Egg Whites

6. Beat the egg whites: In a clean, dry bowl, beat the egg whites using an electric mixer on medium speed until foamy. Add the cream of tartar and continue to beat until soft peaks form.
7. Add sugar gradually: With the mixer running, slowly add 1/3 cup granulated sugar. Increase the speed to high and continue beating until stiff, glossy peaks form.

Combining the Mixtures

8. Fold egg whites into chocolate mixture: Gently fold one-third of the beaten egg whites into the chocolate mixture to lighten it. Carefully fold in the remaining egg whites in two additions, using a spatula to incorporate without deflating the mixture.

Baking the Soufflés

9. Fill the ramekins: Divide the batter evenly among the prepared ramekins, filling each to about 3/4 full.
10. Bake: Place the ramekins on the preheated baking sheet in the oven. Bake for 12-15 minutes, or until the soufflés have risen and the tops are set but still slightly wobbly in the center.