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).

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.

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.