Line an 8-inch round cake pan with plastic wrap, letting excess extend over sides of pan.
In the top of a double boiler, combine chocolate and butter. Cook over simmering water, stirring constantly, until chocolate melts and mixture is smooth. Remove from heat; let cool slightly.
In a medium bowl, beat together egg yolks with a mixer at medium speed until thickened, approximately 1 minute. With mixer running, slowly add ⅓ cup sugar. Increase mixer speed to medium-high, and beat until thickened and ribbons form, 2 to 3 minutes.
In another medium bowl, using clean beaters, beat together egg whites with a mixer at medium speed until foamy, approximately 1 minute. With mixer running, slowly add remaining ⅓ cup sugar. Increase mixer speed to high, and beat until stiff peaks form, 7 to 8 minutes.
In another medium bowl, beat together cream and vanilla extract with a mixer at high speed until stiff peaks form, 2 to 3 minutes.
Fold egg yolk mixture into cooled chocolate mixture until completely combined. Fold in egg white mixture. Fold in whipped cream. Pour batter into prepared pan. Top with Chocolate Sponge Cake, parchment side up. Cover with excess plastic wrap, and freeze until set, at least 6 hours or overnight.
When ready to serve, unwrap and remove parchment from cake layer. Invert onto a serving platter, and gently remove cake pan using plastic wrap to help. Discard plastic wrap. Using hot, dry offset spatula, smooth edges of cake. Dust with cocoa powder. Garnish with chocolate curls and raspberries, if desired.
Notes
*Because the eggs used in this recipe are never cooked, we recommend using pasteurized eggs, such as Davidson’s.
Recipe by TeaTime Magazine at https://teatimemagazine.com/chocolate-mousse-cake-recipe-2/