Stir 2 tablespoons plus 1 ½ teaspoons cornstarch and 2 tablespoons plus 1 ½ teaspoons water together in a small bowl to make a slurry.

Place 9 ounces of the raspberries, ⅓ cup granulated sugar, and ⅛ teaspoon kosher salt in a small saucepan. Bring to a simmer over medium-low heat, mashing the berries with the back of a fork or flexible spatula until completely broken-down, 3 to 6 minutes.

Pour the mixture through a fine-mesh strainer set over a heatproof bowl and press through the strainer; discard the contents of the strainer. Return the strained mixture to the saucepan.

Whisk the slurry into the raspberry mixture. Add the remaining 9 ounces raspberries and stir to combine. Bring to a boil over medium-low heat, stirring often. Continue to boil, stirring often, until thickened and the mixture clings to the back of a spoon without immediately dripping off, 2 to 4 minutes more (175ºF to 180°F).

Transfer the raspberry filling to a heatproof bowl. Refrigerate until fully cooled, at least 1 ½ hours. Meanwhile, make the cake.

Heat the oven to 325°F. Coat 2 (9-inch) round cake pans with cooking spray. Line the bottom of each with a parchment paper round.

Whisk together the sour cream, eggs, 1 ½ cups granulated sugar, neutral oil, light brown sugar, vanilla extract, and kosher salt until smooth.

Sift together the all-purpose flour, cocoa powder, baking powder, and baking soda directly into the bowl and whisk until just combined. Whisk in the hot brewed coffee until smooth.

Divide the batter between the prepared pans (about 25 ounces each). Bake until a toothpick inserted into the center comes out clean, 25 to 35 minutes.

Place the pans on a wire rack and let cool slightly, about 10 minutes. Run a small offset spatula or paring knife around each cake. Flip the cakes out of their pans onto the rack, remove the parchment, and let cool completely, at least 1 hour. Meanwhile, make the frosting.

Place 6 ounces coarsely chopped bittersweet chocolate in a microwave-safe small bowl. Microwave in 30-second intervals, stirring in between each, until melted, 1 to 1 ½ minutes total. Let cool until warm to the touch.

Beat together the unsalted butter, powdered sugar, vanilla extract, and kosher salt in a stand mixer on low speed until just combined, then increase speed and beat until smooth and lightened.

Beat in cold heavy cream or milk on medium-high speed until fluffy, about 2 minutes.

With the mixer on medium speed, pour in the melted chocolate and beat until smooth.

If the cakes are domed, trim the slightly rounded tops as needed. Place one of the cakes cut-side up on a cake board or serving plate.

Stir the raspberry filling and spread it in an even layer over the cake, leaving a ¼-inch border. Place the second cake on top with the flattest side facing up.

Spread a very thin layer of the frosting over the sides of the cake to fill in any gaps and seal in the raspberry filling. Chill until the frosting is set.

Frost the cake with the remaining frosting, smoothing or swooping as desired. Decorate the top with fresh raspberries if desired. Refrigerate before serving.