This famous chocolate sponge cake is filled with apricot jam and coated with silky-smooth ganache. It was created in 1832 by Franz Sacher of the Hotel Sacher, in Vienna.
Cake: Preheat oven to 300 degrees. Butter two 8-inch round cake pans. Line bottoms with parchment; butter parchment. Dust with cocoa powder and tap out excess. Melt chocolate in a bowl set over a pan of simmering water, stirring often; set aside.
Using a mixer, beat together butter and 2/3 cup sugar on medium speed until mixture is pale and fluffy, about 2 minutes. Beat in egg yolks, then vanilla and melted chocolate until thoroughly incorporated. Reduce speed to low and beat in flour. Transfer batter to a large bowl. Put egg whites and salt into cleaned bowl of mixer fitted with whisk attachment. Whisk eggwhite mixture on low speed until foamy. Gradually beat in remaining 1/4 cup sugar and beat just until stiff peaks form, 2 to 3 minutes. Stir half of meringue into chocolate mixture. Gently fold in remaining meringue with a rubber spatula.
Divide batter between pans; tap firmly on counter to remove any air bubbles. Bake until centers are just set, about 25 minutes. Transfer cakes in pans to wire racks and let cool completely, about 1 hour. Turn out cakes, remove parchment, and reinvert onto rack. Trim cakes flat with a serrated knife.
Filling: Warm jam in a small saucepan over medium-low heat, stirring occasionally, until loose. Stir in liqueur. Spread warm jam over 1 cake layer and top with remaining layer.
Ganache (crumb coating): Place chocolate in a large bowl. Bring cream just to a boil over medium-high heat; pour over chocolate and stir until smooth. Refrigerate, stirring every 10 minutes, until ganache is cool and the consistency of buttercream, about 30 minutes. Spread ganache over top and sides of cake with a small offset spatula to create a smooth layer. Refrigerate cake until ganache is firm, about 30 minutes.
Ganache (glaze): Place cake on a rack set over a rimmed baking sheet. Place chocolate in a large bowl. Bring cream just to a boil over medium-high heat; pour over chocolate and let stand 5 minutes. Stir until smooth and the consistency of honey. Slowly pour ganache over top and down sides of cake, spreading evenly with offset spatula if needed. Let stand until ganache loses its sheen, about 1 hour. Using a small, fine sieve, sift luster dust over top of cake. Dusted cake can be made up to 2 days ahead and stored in refrigerator.