Step 1

Heat milk and chocolate in a small saucepan over medium-low, whisking often, until chocolate is melted and mixture is smooth. Cover and chill.

Step 2

Do Ahead: Soak can be made 3 days ahead. Keep chilled.

Step 3

Place ½ cup cold water in a small bowl and sprinkle gelatin evenly over top; chill until ready to use.

Step 4

Place chocolate and salt in a large wide bowl. Heat milk and corn syrup in a small saucepan over medium, whisking to combine, until steaming and barely simmering around the edges of pan. Remove from heat and add softened gelatin, whisking until dissolved. Reheat milk mixture briefly over medium-low (do not let it boil), then pour over chocolate mixture and let sit 1 minute. Gently whisk until smooth; reserve saucepan.

Step 5

Heat cream in reserved saucepan over medium just until warm to the touch, about 1 minute. Gradually stream cream into chocolate mixture, whisking constantly until incorporated and smooth. You should have around 4 cups ganache. Cover and chill until set, at least 4 hours.

Step 6

Do Ahead: Ganache can be made 3 days ahead. Keep chilled.

Step 7

Line a 9"-diameter cake pan or springform pan with plastic wrap, pressing into bottom and leaving generous overhang. Fit a 1-piece round of cake into pan, placing bottom side up. Using a pastry brush, dab a quarter of soak across entire surface of cake. Spread a third of cherry preserves over with a small offset spatula. Stir chilled ganache to loosen, then scrape a third over cherry preserves and smooth into an even layer. Place another round of cake on top of ganache, bottom side up, this time using a 2-piece layer. Repeat soaking and filling process. Top with the remaining 2-piece round of cake, bottom side up, and repeat soaking and filling process one more time. Top with remaining 1-piece round of cake, bottom side up, and brush with remaining soak. You should have 4 layers of soaked cake separated by 3 layers of filling. Wrap plastic overhang up and over cake; chill at least 12 hours and up to 2 days to allow ganache to set and cake to absorb soak.

Step 8

To frost, remove plastic from top of cake and invert cake onto a cake plate or platter. Remove pan and plastic. Dollop about 1 cup buttercream over top of cake and smooth across top and down sides, creating a very thin base layer. You want to fill in any gaps, adhere any crumbs to the cake, and seal in the fillings (this is called a crumb coat). Chill cake 10 minutes, then scrape remaining frosting on top of cake and spread all over, working down and around sides. Decorate as desired.

Step 9

Do Ahead: Cake can be made 2 days ahead. Chill until frosting sets; then cover loosely with plastic wrap. Let sit at room temperature 1–1½ hours before slicing.

Step 10

Highly recommended reading: Layer cake tips, tricks, and common mistakes from Natasha Pickowicz.