Preheat your oven to 400°F (200°C) and place a baking sheet on the middle rack. Generously butter the inside of four 6-ounce ramekins using upward strokes, then coat them evenly with granulated sugar, tapping out any excess.

In a heatproof bowl set over a saucepan of simmering water (a double boiler), melt the dark chocolate and 3 tablespoons of butter. Stir gently until completely smooth and glossy, then remove from heat and let it cool slightly for about 5 minutes.

Once the chocolate mixture has cooled slightly, whisk in the egg yolks one at a time until fully incorporated. Stir in the vanilla extract and set the chocolate base aside.

In a separate, clean glass or metal bowl, beat the egg whites, cream of tartar, and a pinch of salt using an electric mixer on medium speed until soft peaks form. Gradually add the 1/4 cup of granulated sugar, beating on high speed until stiff, glossy peaks form, about 2 to 3 minutes.

Gently fold one-third of the whipped egg whites into the chocolate mixture to lighten it. Carefully fold in the remaining egg whites using a spatula, scraping the bottom and folding over just until no white streaks remain, being careful not to deflate the batter.

Divide the souffle batter evenly among the prepared ramekins, filling them almost to the top. Run your thumb around the inside rim of each ramekin to create a small groove; this helps the souffles rise perfectly straight up.

Place the ramekins on the preheated baking sheet in the oven and immediately lower the temperature to 375°F (190°C). Bake for 12 to 15 minutes without opening the oven door, until the souffles are puffed and the centers are slightly jiggly.

Remove the souffles carefully from the oven and serve immediately, as they will begin to deflate within minutes. Dust with powdered sugar and serve alongside a scoop of vanilla bean ice cream for the ultimate dessert experience.