Preheat oven to 350°F (175°C); grease an 8-inch round springform pan and line bottom with parchment paper.
Melt chopped bittersweet chocolate and butter in a heatproof bowl set over simmering water; cool slightly.
Whisk egg yolks, 3 tablespoons granulated sugar, and vanilla extract into melted chocolate.
In a clean bowl, beat egg whites with remaining granulated sugar until stiff glossy peaks form.
Fold whipped egg whites into chocolate mixture gently in three additions.
Spread batter into prepared springform pan; bake for 16 to 18 minutes until set; cool completely in pan.
Puree raspberries in a blender and strain through a fine mesh sieve to remove seeds.
Melt bloomed gelatin in microwave for 15 seconds; whisk into raspberry puree.
Whip cold heavy cream with powdered sugar to medium peaks; gently fold in raspberry mixture to create mousse.
Pour raspberry mousse over cooled chocolate cake in springform pan; smooth top and chill for 4 hours.
Pour warm chocolate ganache over set mousse; chill for 30 minutes; release springform ring and top with fresh raspberries.
