Preheat your oven to 350 degrees F. Start by lining the bottom of two 8-inch cake pans with parchment paper and spraying them with nonstick cooking spray. Make sure to get the sides as well, so the cake releases easily after baking.
In a medium bowl, sift together 1 3/4 cups of all-purpose flour, 3/4 cup of dark unsweetened cocoa powder, 1 teaspoon of baking powder, and 2 teaspoons of baking soda. Add in 2 cups of granulated sugar and 1 teaspoon of salt; whisk everything together until well combined. Set this dry mixture aside.
In a large bowl, whisk together 1 cup of buttermilk (make sure it’s at room temperature), 1/2 cup of vegetable oil, 2 large eggs, and 2 teaspoons of vanilla extract until everything is nicely blended.
Gradually add the dry ingredients to the wet ingredients, mixing gently until just combined. Be careful not to overmix; the batter will be thin, but that’s okay!
Now, carefully whisk in 3/4 cup of boiling water until smooth. This is what helps create a moist cake, so don’t skip this step!
Evenly divide the batter between the prepared cake pans. Tap the pans on the counter a few times to release any air bubbles that may have formed.
Bake in the preheated oven for approximately 32 to 38 minutes for the 8-inch pans, or 22 to 28 minutes for 9-inch pans. Keep an eye on your cake, and check for doneness by inserting a toothpick into the center; it should come out with just a few moist crumbs.
Once done, cool the cakes in the pans for about 10 minutes before transferring them to a wire rack to cool completely.
While the cakes cool, let's prepare the raspberry filling. In a food processor, combine 12 ounces of thawed frozen raspberries and any juices. Puree the mixture until smooth, then strain it through a fine mesh sieve to remove the seeds, using the back of a spoon to press it through.
In a small saucepan over medium-high heat, combine the raspberry puree with 1 1/2 tablespoons of cornstarch, 2 tablespoons of lemon juice, and 1/3 cup of granulated sugar. Stir constantly until it thickens, then refrigerate until completely chilled.
Next, prepare the chocolate ganache. In a large microwave-safe bowl, heat 1 2/3 cups of heavy cream in the microwave until just boiling (about 2 to 3 minutes). Remove from the microwave and immediately add 12 ounces of chopped bittersweet chocolate. Let it sit for one minute, then whisk vigorously until smooth.
Take out a heaping 1/2 cup of ganache to use later for the chocolate mascarpone filling. Set aside the remaining ganache to cool until barely lukewarm and spreadable.
For the chocolate mascarpone, in a bowl, beat together 6 ounces of mascarpone, 1 1/4 cups of heavy cream, 1/2 cup plus 1 tablespoon of confectioners' sugar, and 3/4 teaspoon of vanilla until thick and creamy.
Now it’s time to assemble! Cut each cooled cake in half horizontally, creating four layers. On a serving plate, place one layer, cut side up, and add one-third of the raspberry filling. Top with one-third of the chocolate mascarpone.
Repeat these layers with the remaining cake layers, ending with the last layer cut side down on top.
Frost the top and sides of your cake with a thin layer of ganache for a crumb coat. Freeze the cake for 30 minutes to set this layer.
After freezing, frost the cake with the remaining ganache. If the ganache is too thick, you can microwave it for just a few seconds to soften it.
Decorate the top with fresh raspberries arranged beautifully, and dust lightly with powdered sugar before serving. If you’re not serving immediately, cover the cake with an inverted bowl or cake cover and refrigerate. Let it come to room temperature before serving.
