Step 1

Preheat oven to 350°. Coat cake pans with nonstick spray. Line bottom of pans with parchment-paper rounds; coat paper. Place cocoa powder and chocolate in a medium metal bowl. Pour hot coffee over. Let stand for 1 minute. Stir until smooth. Stir in buttermilk and vanilla; set aside.

Step 2

Whisk cake flour and next 3 ingredients in a medium bowl. Using an electric mixer, beat sugar and butter in a large bowl until light and fluffy, about 4 minutes. Add eggs one at a time, beating to blend between additions and occasionally scraping down sides and bottom of bowl. Beat until light and fluffy, about 4 minutes. Add dry ingredients in 3 additions, alternating with chocolate mixture in 2 additions, beginning and ending with dry ingredients. Divide batter evenly between pans; smooth tops.

Step 3

Bake cakes until a tester inserted into the centers comes out clean, about 35 minutes. Transfer to wire racks; let cakes cool in pans for 10 minutes (cakes will deflate slightly). Run a knife around pans to loosen cakes; invert cakes onto racks. Peel off paper and let cakes cool completely. Turn cakes over.

Step 4

If needed, use a long serrated knife to cut off bumps or trim dome from top of each cake to create a flat, even surface.

Step 5

Place chocolate and salt in a medium bowl. Stir sugar and 1/4 cup water in a medium deep saucepan over medium-low heat until sugar dissolves. Increase heat and cook without stirring, occasionally swirling pan and brushing down sides with a wet pastry brush, until sugar is deep amber, about 9 minutes. Remove from heat and gradually add cream (mixture will bubble vigorously). Stir over medium heat until caramel bits dissolve. Pour over chocolate in bowl. Add vanilla; stir until mixture is smooth. Let cool slightly.

Step 6

Place 1 cake layer in springform pan. Pour 1 cup ganache over. Chill until set, about 30 minutes. Cover remaining ganache and let stand at room temperature.

Step 7

Place 2 Tbsp. cold water in a small heatproof glass or metal bowl. Sprinkle gelatin over; let stand until gelatin softens, about 10 minutes.

Step 8

Pour water to a depth of 1/2-inch into a small skillet set over medium heat. Transfer bowl with gelatin to skillet; stir until gelatin dissolves, about 2 minutes. Remove bowl from skillet. Set aside.

Step 9

Place cream and powdered sugar in a large bowl. Scrape in seeds from vanilla bean. Using an electric mixer, beat cream until soft peaks form. Add gelatin; beat filling until firm peaks form.

Step 10

Spoon filling over chilled ganache on cake layer in pan; smooth top. Gently place second cake layer on top. Cover tightly with plastic wrap and chill until cream layer is set, at least 6 hours or overnight.

Step 11

Remove sides from springform pan. Using a knife or offset spatula, scrape off any filling that may have leaked out from between cakes to form smooth sides. Transfer cake to a wire rack set inside a rimmed baking sheet.

Step 12

Rewarm remaining ganache until just pourable. (Microwave in a microwave-safe bowl, or set a metal bowl over a large saucepan of simmering water until just warm, not hot.) Pour ganache over cake, tilting cake as needed to allow ganache to drip down sides and using an offset spatula to help spread ganache, if needed, to cover sides of cake. Chill until ganache is set, about 1 hour.

Step 13

DO AHEAD: Cake can be made 2 days ahead. Cover with a cake dome; chill. Let stand at room temperature for 1 hour before serving.

Step 14

Sprinkle cake with flaky sea salt.