Set a rack in the lower third of the oven and heat it to 400°F (200°C). Place a baking sheet on the rack so it heats too. Brush the softened butter inside each ramekin with straight, upward strokes, which helps the soufflé climb. Add 1/2 tablespoon sugar to each ramekin, turn to coat, and tap out the excess. Chill the ramekins while you work.

Melt the chocolate and the 3 tablespoons of butter in a heatproof bowl over barely simmering water. Stir until glossy and smooth. Remove from the heat and let it cool for 3 to 4 minutes. It should feel warm, not hot, to the back of a finger. Whisk in the egg yolks one at a time, then the vanilla and salt. The mixture will look thick and shiny.

Use a clean, dry bowl, because any trace of fat or yolk will stop the whites from whipping. Beat the whites with the cream of tartar on medium speed until foamy. Add the 1/4 cup sugar one spoonful at a time, then raise the speed. Stop at medium-stiff peaks. The whites should look glossy, and the peak should hold but curl slightly at the tip. Overbeaten whites turn grainy and will not fold in smoothly.

Stir about one-third of the whites into the chocolate to loosen it. Add the remaining whites in two batches. Use a wide spatula, cutting down through the center, sweeping along the bottom, and turning the bowl as you go. Stop when you see no white streaks. A few tiny bubbles are fine, but do not stir.

Spoon the batter into the ramekins, filling them to the rim. Level the tops with an offset spatula or a knife. Run your thumb around the inside edge to make a small gap, which gives the soufflé a clean rise. Set the ramekins on the hot baking sheet and bake for 12 to 14 minutes. The soufflés are done when they have risen about 1 inch (2.5 cm) above the rim, the tops are set, and the centers jiggle slightly when you nudge the pan. Do not open the oven before the 10-minute mark.

Dust with powdered sugar and bring the soufflés to the table at once. They begin to fall within 2 to 3 minutes, which is normal. Break the top with a spoon and add your sauce or ice cream. Rebecca's Tip: You can fill the ramekins up to 1 hour ahead and keep them at cool room temperature. Add 1 to 2 minutes to the bake time. Making the batter a little ahead is the easiest way to serve soufflé without stress at a dinner party.