Preheat your oven to 350°F (175°C). Grease an 8-inch round springform pan and line the bottom with parchment paper, then dust the sides lightly with a little cocoa powder to prevent sticking.

Place the chopped dark chocolate and cubed butter in a heatproof bowl set over a saucepan of simmering water, creating a double boiler. Stir gently until completely melted and smooth, then remove from heat and let cool slightly for about 5 minutes.

In a large mixing bowl, whisk the egg yolks with 1/2 cup of the granulated sugar until the mixture becomes pale and slightly thickened, about 2 minutes. Stir in the vanilla extract and the slightly cooled chocolate mixture until well combined.

In a separate clean glass or metal bowl, use an electric mixer to beat the egg whites and salt on medium speed until soft peaks begin to form. Gradually add the remaining 1/4 cup of sugar, beating on high speed until stiff, glossy peaks form.

Gently fold one-third of the beaten egg whites into the chocolate mixture to lighten the batter. Carefully fold in the remaining egg whites in two batches, using a gentle over-and-under motion to avoid deflating the batter, until no white streaks remain.

Pour the delicate batter into the prepared springform pan and smooth the top with a spatula. Bake for 35 to 40 minutes, or until the edges are set and the center has a slight jiggle when gently shaken.

Let the cake cool completely in the pan on a wire rack; it will sink slightly in the center as it cools, creating a beautifully rustic, cracked top. Run a thin knife around the edges before removing the springform ring, then dust generously with cocoa powder and serve with fresh raspberries.