01 - Preheat oven to 400°F. Generously butter four 6-ounce ramekins and dust with granulated sugar, tapping out any excess.
02 - In a heatproof bowl set over a pot of simmering water, melt the chopped dark chocolate and 2 tablespoons of butter, stirring until completely smooth. Remove from heat and allow to cool slightly.
03 - In a large mixing bowl, whisk the egg yolks with half of the granulated sugar (2 tablespoons) until the mixture is pale and thick. Stir in the pure vanilla extract and a pinch of salt. Gradually add the cooled melted chocolate mixture and all-purpose flour, mixing until just combined.
04 - In a separate, clean bowl, beat the egg whites until soft peaks begin to form. Gradually add the remaining 2 tablespoons of granulated sugar and continue beating until glossy, stiff peaks are achieved.
05 - Gently fold one-third of the whipped egg whites into the chocolate mixture to lighten the batter. Carefully fold in the remaining egg whites until they are just incorporated, being careful not to deflate the mixture.
06 - Divide the soufflé batter evenly among the prepared ramekins. Run your thumb around the inside edge of each ramekin to create a clean separation, which encourages an even rise.
07 - Place the filled ramekins on a baking sheet. Bake in the preheated oven for 14–18 minutes, or until the soufflés are well risen and appear barely set in the center.
08 - While the soufflés are baking, prepare the raspberry sauce. In a small saucepan, combine the fresh or frozen raspberries, 2 tablespoons granulated sugar, 1 teaspoon lemon juice, and 1 tablespoon water (if using frozen berries). Cook over medium heat for 3–4 minutes, or until the raspberries break down and release their juices. Strain the sauce through a fine mesh sieve to remove any seeds, then press the solids to extract all liquid. The sauce can be served warm or at room temperature.
09 - Dust the baked soufflés with powdered sugar immediately after removing them from the oven. Serve at once with the prepared raspberry sauce on the side.