Preheat your oven to 350°F (175°C) and prepare a 9-inch springform pan by greasing it and lining the bottom with parchment paper.
In a double boiler, melt the bittersweet chocolate and unsalted butter together, stirring until smooth. Remove from heat and allow to cool slightly.
In a separate bowl, using an electric mixer, whisk the eggs until foamy. Gradually add the sugar and continue to whisk until the mixture is thick and pale.
Gently fold the egg mixture into the melted chocolate in three additions, being careful not to deflate the eggs. Add the vanilla extract and salt, and mix until fully combined.
Pour the batter into the prepared springform pan and bake for 25-30 minutes, or until the center is set. Allow the cake to cool in the pan for 10 minutes before removing the sides of the springform.
Refrigerate the cake for at least 8 hours or overnight to enhance the flavor and texture.
For the ganache, heat the heavy cream in a small saucepan until just simmering. Pour the hot cream over the remaining bittersweet chocolate (about 4 ounces) in a bowl and let sit for 10-15 minutes. Stir until smooth and pour over the chilled cake.
Garnish with fresh raspberries before serving.