Preheat your oven to 375°F (190°C). Grease a 1½-quart soufflé dish with butter and dust lightly with sugar or cocoa powder.

In a saucepan, melt 2 tablespoons of butter over medium heat. Add the flour and whisk for 1 minute to form a roux.

Gradually whisk in the milk and stir until smooth and thick, about 2–3 minutes.

Remove from heat and add the chopped chocolate. Stir until fully melted and smooth.

Mix in the vanilla, salt, and egg yolks one at a time. Set aside to cool slightly.

In a clean bowl, beat the egg whites and cream of tartar until soft peaks form. Gradually add sugar and beat to stiff, glossy peaks.

Stir a spoonful of the egg whites into the chocolate mixture to lighten it. Then gently fold in the rest, being careful not to deflate the batter.

Pour the mixture into the prepared soufflé dish. Run your thumb around the inside edge of the dish to help the soufflé rise evenly.

Bake for 25–30 minutes, until puffed and slightly jiggly in the center.

Serve immediately with a dusting of powdered sugar if desired.