Make the cake: Heat oven to 350°. Butter and flour a 3″-deep 9″ cake pan; set aside. Combine 1 cup sugar and eggs in the bowl of a stand mixer fitted with a whisk; beat on medium-high speed until tripled in volume, about 8 minutes. Whisk together flour, cornstarch, cocoa powder, and salt in a bowl; sift onto egg mixture. Add butter; working quickly, gently fold until just combined. Pour into prepared pan; smooth top with a rubber spatula. Bake until a toothpick inserted in center of cake comes out clean, about 45 minutes; let cool. Using a serrated knife, trim top of cake to create a level top; cut cake horizontally into 3 even layers (see How to Cut Cake Layers) and set cakes aside.
Make the kirsch syrup: Stir together 14 oz. cherries and kirsch in a bowl; let sit for 30 minutes. Bring remaining sugar and reserved cherry juice to a boil in a 1-qt. saucepan and stir until sugar dissolves; remove from heat and set aside. Drain kirsch from cherries, and add it to cherry syrup in saucepan. Set soaked cherries and syrup aside.
Make the frosting: Sprinkle gelatin over 6 tbsp. milk in a bowl; let sit until gelatin softens, about 5 minutes. Whisk cornstarch and sugar in a 2-qt. saucepan; add remaining milk, and heat over medium heat. Cook, stirring constantly, until mixture thickens to the consistency of very thick pudding; transfer to a food processor along with gelatin mixture; process until smooth. Transfer gelatin mixture to a large bowl. Place cream, 4 tbsp. kirsch, and vanilla in the bowl of a stand mixer fitted with a whisk; beat on medium-high speed until stiff peaks form. Add 1⁄3 of the whipped cream to the gelatin mixture; stir until smooth. Add remaining whipped cream, and gently fold with a rubber spatula until evenly combined.
To assemble the cake: Place 1 layer of cake on a cake stand; brush heavily with cherry syrup. Cover with soaked cherries; spread 1 cup frosting over cherries. Place another cake layer on top; brush heavily with syrup. Spread 2 cups frosting over cake; top with remaining cake layer and brush heavily with syrup. Spread a very thin layer of frosting over top of cake; using a long knife, score top into 12 wedges. Transfer remaining frosting to a piping bag fitted with a 3⁄8″ star tip; starting at outside edge of top of cake, pipe zig-zag rows of frosting on wedges, staying between scored lines. Pipe 3 rows of frosting around side of cake; smooth rows until frosting evenly covers side. Drag a 6" x 4" decorating comb (available at Kerekes) over side of cake to produce a ridged pattern. Pipe 12 small mounds of frosting on top of each wedge; place a cherry on top of each mound. Pour chocolate shavings over center of cake. Chill until frosting is firm, about 2 hours. To serve, pour remaining 2 tbsp. kirsch over chocolate.