Preheat your oven to 400°F (200°C) and place a baking sheet on the middle rack. Generously grease four 6-ounce ramekins with softened butter, brushing in upward strokes to encourage the souffles to rise. Coat the insides entirely with granulated sugar, tapping out any excess.

Place the chopped dark chocolate and 4 tablespoons of butter in a heatproof bowl set over a saucepan of gently simmering water. Stir occasionally until the mixture is completely melted, smooth, and glossy. Remove the bowl from the heat and stir in the vanilla extract.

Allow the chocolate mixture to cool slightly for about 5 minutes so it doesn't scramble the eggs. Whisk the egg yolks into the chocolate one at a time until the mixture is smooth and fully incorporated.

In a perfectly clean, grease-free glass or metal bowl, beat the egg whites, cream of tartar, and a pinch of salt with an electric mixer on medium speed until soft peaks form. Gradually sprinkle in the 1/4 cup of granulated sugar while continuing to beat until stiff, glossy peaks form.

Take one-third of the whipped egg whites and gently stir it into the chocolate mixture to lighten the base. Carefully fold in the remaining egg whites in two additions using a rubber spatula, slicing down the middle and folding over to keep the batter airy, stopping as soon as no white streaks remain.

Divide the batter evenly among the prepared ramekins, filling them completely to the top. Use a flat knife to scrape the tops level, then run your thumb around the inner rim to create a shallow groove, which helps the souffle rise straight up.

Place the ramekins on the preheated baking sheet in the oven and bake for 12 to 15 minutes, or until puffed and set on the outside but still slightly jiggly in the center. Serve immediately with a dusting of powdered sugar and enjoy right away before they lose their height.