This towering cake melds rich bittersweet ganache, creamy milk-chocolate mousse, and multiple layers of chocolate sponge cake. Adorned with buttery brittle, it can't help but set hearts aflutter.
Make cake: Preheat oven to 350 degrees. Butter an 8-inch round cake pan. Line with parchment, and butter parchment. Sift together flour, cocoa, and salt.
With a mixer fitted with the whisk attachment, beat egg yolks and sugar on medium-high speed until pale and thick, about 7 minutes. Beat in vanilla.
With a mixer fitted with the whisk attachment, beat egg whites until medium-stiff peaks form. Fold into yolk mixture in 3 additions. Fold in flour mixture in 2 additions until almost incorporated. Fold in butter. Pour into pan.
Bake cake until a toothpick inserted into center comes out clean, 25 to 30 minutes. Let cool completely in pan on a wire rack. Invert onto rack. Remove parchment, and turn right side up.
Make brittle: Bring sugar, corn syrup, water, and salt to a boil, stirring until sugar has dissolved. Wash down sides of pan with a wet pastry brush, and cook until pale amber, about 12 minutes total. Stir in butter. Remove from heat, and stir in cashews. Pour onto a parchment-lined baking sheet to cool. Coarsely chop.
Make mousse: Melt chocolates and butter in a heatproof bowl set over simmering water, stirring occasionally. Cool 20 minutes. Keep water simmering.
Combine yolks, sugar, and salt in a heatproof bowl set over simmering water. Whisk until sugar has dissolved and mixture reaches 160 degrees on a candy thermometer. Remove from heat. With a mixer fitted with the whisk attachment, beat on medium-high speed until thick and cool, about 7 minutes.
Whisk cream, creme fraiche, and vanilla until medium-stiff peaks form. Fold chocolate mixture into yolks. Fold in cream mixture, and then 1 1/2 cups brittle. Cover, and refrigerate, stirring occasionally, until slightly thickened, 1 hour.
Make ganache: Fill a bowl halfway with ice water, and set aside. Combine chocolates and salt in a heatproof bowl. Bring milk, cream, and butter to a boil. Pour over chocolate, and whisk until smooth. Reserve 1 cup ganache at room temperature. Place bowl with remaining ganache in ice water, and whisk constantly, scraping bowl, until ganache is set but not solid.
Make syrup: Mix liqueur and vanilla.
Assemble cake: Using a serrated knife, trim rounded top of cake to create a flat surface, and shave off just the slightest bit of cake around edges so that it fits inside an 8-by-3-inch springform pan without touching sides. Cut cake in half horizontally. Cut both layers in half horizontally to make 4 layers. Place 1 layer in pan. Brush with 1/4 of the syrup. Top with 1/2 cup ganache and 1 cup mousse. Repeat with cake, syrup, ganache, and mousse. Spread 1 cup reserved ganache over top. Refrigerate at least 2 hours (preferably overnight).
Run an offset spatula around edges of pan, and remove sides. Garnish with berries and, if desired, Cashew-Brittle Hearts.