Prep the oven and ramekins: Heat oven to 375°F (190°C). Generously grease 4–6 small ramekins with butter, then coat the insides with unsweetened cocoa powder. Tap out the excess. This helps the soufflés climb.

Melt the chocolate: In a heatproof bowl, melt chopped dark chocolate with butter (microwave in short bursts or use a double boiler). Stir until smooth. Let it cool for a couple of minutes so it’s warm, not hot.

Make the base: Whisk in heavy cream, vanilla, a pinch of salt, and half of the granulated keto sweetener. If using, add a pinch of espresso powder. Then whisk in the egg yolks until glossy and combined.

Whip the egg whites: In a clean bowl, beat egg whites with a pinch of salt and cream of tartar (if using) to soft peaks. Slowly rain in the remaining keto sweetener. Continue beating to medium-stiff peaks. They should look glossy and hold lines.

Lighten the batter: Stir a big spoonful of whipped whites into the chocolate base to loosen it. Gently fold in the rest with a spatula, working in thirds. Use light, sweeping motions to avoid deflating.

Fill the ramekins: Spoon the batter in, almost to the top. Run your thumb around the inside rim of each ramekin to create a small moat. This encourages an even rise with a proud top.

Bake: Place ramekins on a baking sheet. Bake 10–14 minutes, depending on size. Look for a tall rise with a slight wobble in the center. Don’t open the oven early.

Serve immediately: Soufflés start to fall as they cool. Dust with powdered keto sweetener or add a spoon of lightly sweetened whipped cream. Enjoy warm and airy.