Preheat your oven to 400°F (200°C). Generously butter the inside of four 6-ounce ramekins, then coat them completely with granulated sugar, tapping out any excess. This sugar coating gives the souffle something to grip as it rises.

Create a double boiler by placing a heatproof bowl over a pot of simmering water, ensuring the bottom of the bowl does not touch the water. Add the chopped dark chocolate and 3 tablespoons of butter, stirring occasionally until completely melted and smooth. Remove from heat and stir in the vanilla extract.

Allow the chocolate mixture to cool slightly for about 5 minutes. Whisk the egg yolks into the warm chocolate one at a time, ensuring each is fully incorporated before adding the next, until the mixture is thick and glossy.

In a separate, perfectly clean glass or metal bowl, beat the egg whites, salt, and cream of tartar using an electric mixer on medium speed. Once soft peaks begin to form, gradually add the 1/4 cup of granulated sugar, beating until stiff, glossy peaks form.

Gently fold one-third of the egg white mixture into the chocolate base to lighten it. Then, carefully fold in the remaining egg whites using a rubber spatula, scraping the bottom and sides of the bowl until no white streaks remain, being careful not to deflate the batter.

Divide the mixture evenly among the prepared ramekins, filling them almost to the top. Run your thumb around the inside edge of each ramekin to create a small channel; this helps the souffles rise straight up and prevents lopsided tops.

Place the ramekins on a baking sheet and bake for 12 to 15 minutes, or until puffed and set around the edges but still slightly jiggly in the center. Do not open the oven door during the first 10 minutes of baking, or the sudden draft may cause them to fall.

Remove the souffles from the oven and immediately dust with powdered sugar using a fine mesh sieve. Serve right away with a dollop of whipped cream or vanilla ice cream, as they will begin to deflate within minutes of leaving the heat.