Brush inside of four 5-ounce or three 6-ounce soufflé ramekins with 2 tablespoons of butter. Pour 3 tablespoons of sugar into 1 prepared ramekin, coating the inside with sugar. Repeat until all ramekins are coated. Refrigerate prepared ramekins.

Preheat the oven to 400F.

In a small saucepan, bring 2 inches of water to a boil. Reduce heat to medium-low. Place a heat-proof bowl over the simmering water, ensuring it does not touch the water. Add chocolate and remaining 3 tablespoons of butter to the bowl. Stir until melted and smooth. Remove from heat and let cool for 2 minutes.

Whisk in egg yolks, vanilla extract, and salt until smooth. Set aside.

In a large mixing bowl, beat egg whites and cream of tartar on medium-high speed until foamy and doubled in size, about 1 to 2 minutes. Slowly add the remaining 3 tablespoons of sugar while mixing. Continue until stiff peaks form, about 3 more minutes.

Fold the egg whites into the chocolate mixture in 3 batches. Refrigerate the batter for 10 minutes.

Divide the mixture evenly among the prepared ramekins and smooth with an offset spatula. Create a channel around the edge of the batter with an offset spatula or butter knife.

Bake for 12 to 14 minutes or until the batter rises above the rim of the soufflé and the center jiggles when shaken. Dust with confectioners' sugar and serve immediately.