The chocolate cake: Butter and flour 2 (8-inch) round cake pans and preheat oven to 350°F. Sift together cocoa and flour into a bowl; set aside.
Using an electric mixer, whisk together eggs and sugar. Set bowl over a saucepan with an inch of barely simmering water and whisk constantly until eggs are warm to touch, about 2 minutes. Return bowl to mixer and beat at medium-high speed until eggs quadruple in volume, about 12 minutes.
Beat vanilla and salt into egg mixture at medium speed. Sift a third of reserved cocoa mixture on top; fold in until just incorporated. Repeat twice more with remaining cocoa mixture. Fold in melted butter in two parts. Do not overmix.
Gently scrape batter into prepared pans and bake until a cake tester comes out clean, about 20 minutes. Let layers cool in pans for 10 minutes; turn out onto racks to cool completely.
The sweet cherry filling: Reserve 1/4 cup cherry juice from cherries, placing 2 Tbsp. into each of 2 small cups. Place remaining cherries and juice in a small saucepan; stir in 7 Tbsp. sugar, cornstarch and salt. Bring to a boil, stirring; then reduce heat and simmer, stirring, until mixture is thick, about 3 minutes.
Stir remaining 2 Tbsp. sugar into 1 portion of reserved cherry juice and set aside.
The cherry-ganache filling: Combine chocolate and 1/4 cup heavy cream in a small bowl. Microwave at medium power for 2 minutes. Stir until smooth. Transfer mixture to a large metal bowl and whisk in remaining 1 cup cream and reserved 2 Tbsp. cherry juice. Refrigerate for 30 minutes.
Add confectioners' sugar and vanilla to chilled cherry ganache. Beat with an electric mixer until fluffy and thick enough to spread.
The whipped-cream frosting: In a large, chilled, metal bowl, beat cream, sugar and vanilla with an electric mixer until stiff.
The cake assembly: Using a serrated knife, split each cake layer in half to make 4 layers. Place 1 layer on a platter. Spread half of sweet cherry filling on top; cover with a second cake layer. Drizzle half of reserved cherry juice mixture on top and then cover with half of cherry ganache. Repeat layering with cake, cherry filling, cake, cherry juice and ganache.