For the batter: Melt the 4 Tablespoons of butter and the chopped chocolate together in a double boiler or in a large heatproof bowl in the microwave. Microwave in 20 second increments, stirring after each until completely melted and combined. Slightly cool for 3-5 minutes.Whisk egg yolks, vanilla extract, and salt into chocolate mixture. Set aside.In a completely clean residue-free medium glass or metal mixing bowl, using a handheld mixer or stand mixer fitted with a whisk attachment, beat the egg whites and cream of tartar together on high speed until soft peaks form, about 2 minutes. With the mixer running on high speed, slowly add the sugar in 3 additions about 1 Tablespoon at a time, beating for 5 seconds before adding the next Tablespoon. After all 3 Tablespoons of sugar have been added, beat until stiff glossy peaks form, about 2 more minutes. Do not over-beat.Slowly and gently fold the egg whites into the chocolate mixture. It’s best to do this in 3 separate additions, combining each addition completely before folding in more.Refrigerate the batter for 5-10 minutes as you preheat the oven and prepare the ramekins. (No need to cover the batter unless you are refrigerating it for longer than 1 hour. Can be made up to 2 days in advance, see make ahead instructions below.)Adjust the oven rack to the lower third position and preheat to 400°F (204°C).Prepare the ramekins: Brush four 6-ounce oven-safe straight sided ramekins with a generous coating of the softened butter. Sprinkle in the granulated sugar, then shake/rotate each to ensure every spot of the ramekin has a sugar coating. Place ramekins on a baking sheet. Discard excess sugar.Spoon batter evenly into ramekins. Using a knife or icing spatula, smooth down the surface. Run a knife, icing spatula, or your thumb around the rim at the top, creating a “channel” between the batter and the rim of your pan. Feel free to wipe the edges clean with a towel, though I don’t always do that and don’t notice a difference in appearance or rise.Bake: Place baking sheet with ramekins on top in the oven. Immediately reduce oven temperature to 375°F (191°C). Bake for 13-14 minutes or until the edges are set and the center just barely jiggles when you give the pan a light tap. Avoid opening/closing the oven too much.Remove from the oven and serve immediately plain or with optional toppings. Soufflés begin to fall within minutes. You’ll still have that sponge-like texture underneath a crackly top, though it settles into a denser texture as time passes. Cover and store leftovers in the refrigerator for up to 3 days.Chocolate: For the best results, use a 4 ounce “baking chocolate” bar found in the baking aisle. I prefer Bakers or Ghirardelli brands. You can use semi-sweet, bittersweet, or even milk chocolate. We have not tested it with white chocolate, so we can’t guarantee results. Do not use chocolate chips, as they contain stabilizers preventing them from melting into the proper consistency.Eggs: Room temperature egg whites hold more volume, so if you can bring them to room temperature before beginning, that’s helpful. However, I’ve made soufflé with cold eggs and it’s never been a real issue.Cream of Tartar: We only use a small amount of cream of tartar, but it has a BIG job– it helps the egg whites hold their peaks. It’s an imperative ingredient that you cannot skip. In a pinch, the same amount of lemon juice or white vinegar can be used as a substitute. But for best results, use cream of tartar.Other Dishes/Pans: See Pan Options & Bake Times section above. Oven temperatures remain the same no matter which pan/dish you use.Doubling This Recipe: You can double this recipe for 8 individual soufflés by doubling all of the ingredients. Beating the egg whites may take longer since you’re working with more volume.Notes Make Ahead Instructions: You can prepare the souffle batter up to 2 days in advance (steps 1-4). Cover and refrigerate the batter in the mixing bowl or in the ramekins/dish until ready bake. Add another minute or so to the bake time if you refrigerated it for longer than 1 hour. I do not recommend freezing this soufflé batter or the baked and cooled soufflés.Special Tools (affiliate links): Glass Mixing Bowls | Whisk | Egg White Separator | 6 Ounce Oven-Safe Ramekins like these or these | Flat Icing Spatula | Double Boiler (optional)