Whisk the cornstarch and water together until all the cornstarch has dissolved. (I just use a fork to mix—very easy.) Combine cornstarch mixture, raspberries (no need to thaw if using frozen), granulated sugar, and lemon juice in a medium saucepan set over medium heat. Using a silicone spatula, stir the mixture, mashing the raspberries as they begin to thaw and soften. Bring to a boil and let it boil for 5 full minutes, stirring occasionally. Remove pan from heat and stir in vanilla extract.

Allow the raspberry filling to cool at room temperature for 10–15 minutes, then transfer it to a bowl or container and place it in the refrigerator for at least 4 hours and up to 1 week (the longer, the better). No need to cover it, but if refrigerating for longer than 4 hours, cover tightly. It will continue to thicken up as it chills. Raspberry filling must be completely chilled before using in your cake. If freezing, see Note below for instructions.

Preheat oven to 350°F (177°C). Grease three 9-inch cake pans, line with parchment paper rounds, then grease the parchment paper. Parchment paper helps the cakes seamlessly release from the pans. (If it’s helpful, see this parchment paper rounds for cakes video & post.)

Whisk the flour, cocoa powder, sugar, baking soda, baking powder, salt, and espresso powder (if using) together in a large bowl. Set aside. Using a handheld or stand mixer fitted with a whisk attachment (or you can use a whisk), mix the oil, eggs, and sour cream together on medium-high speed until combined. Add the buttermilk and vanilla and beat until combined. Pour the wet ingredients into the dry ingredients, add the hot water/coffee, and whisk or beat on low speed until the batter is completely combined. Fold in the flour-coated chocolate chips. Batter is thin and you may see some air bubbles on the surface—that’s normal. You should have about 6–6.5 cups of batter, or around 1400g.

Divide batter evenly between 3 pans. Bake for approximately 24–26 minutes. Baking times vary, so keep an eye on yours. The cakes are done when a toothpick inserted in the center comes out clean.

Remove the cakes from the oven and set on a wire rack. Allow to cool completely in the pan. The cakes may slightly sink in the middle as they cool—that’s expected.

With a handheld or stand mixer fitted with a paddle or whisk attachment, beat the butter on medium speed until creamy, about 2 minutes. Add confectioners’ sugar, cocoa powder, heavy cream, salt, and vanilla extract. Beat on low speed for 30 seconds, then increase to high speed and beat for 1 full minute. Taste. Beat in another pinch of salt if desired. (Do you want your buttercream darker in color? I have a trick detailed on the full chocolate buttercream page.) You’re going to use this buttercream for a thin layer under the raspberry filling, a piped “dam” around 2 of the cake layers, and for the crumb coat. Makes about 2.5 cups total.

Place 1 cooled cake layer on your cake stand or serving plate. Using a large icing spatula or small offset spatula, evenly cover the top with about 1/4 cup of chocolate buttercream frosting (a thin layer). Spoon about 1/2 cup of the chocolate buttercream into a piping bag fitted with a large round piping tip. I use Wilton 2A. (Or just use a disposable piping bag and cut about 3/4 inch off the tip and use that without a piping tip.) Pipe a thick border of buttercream around the edge of the frosted cake layer, using about half of the buttercream in the piping bag. Then, using a small offset spatula, spread half of the thickened and chilled raspberry filling (about 1/2 cup) inside the buttercream border. Place second cake layer on top and then repeat the filling process: spread frosting, pipe border with remaining frosting in piping bag (if you ran out, just use more from the big bowl of buttercream), then spread on remaining raspberry filling. Top with third cake layer.

Using the remaining chocolate buttercream, spread a thin layer of buttercream on the top and around the sides as a crumb coat. Run a bench scraper around the cake to smooth out crumb coat. Chill uncovered in the refrigerator for at least 30 minutes and up to 3 hours to set the crumb coat.

Place finely chopped chocolate in a medium heat-proof bowl. Heat the cream and raspberry liqueur, if using, in a small saucepan over medium heat until it begins to gently simmer. (Do not let it come to a rapid boil—that’s too hot!) Pour over chocolate, then let it sit for 2–3 minutes to gently soften the chocolate. With a metal spoon or small rubber spatula, very slowly stir until chocolate has melted and mixture is smooth. Ganache is thin. The finer you chopped the chocolate, the quicker it will melt with the cream. If it’s not melting, do not microwave it. If needed, see Troubleshooting Chocolate Ganache. Once ganache mixture is smooth, let it chill for 30 minutes in the refrigerator to thicken before spreading on chilled crumb-coated cake.

Pour/spoon thickened ganache on chilled cake, and spread all over cake with an icing spatula. Garnish with fresh raspberries, if desired. Serve cake immediately or chill, uncovered, for up to 4–6 hours before serving. Cake can be served at room temperature or chilled.

Cover leftover cake tightly and store in the refrigerator for 5 days. I like using a cake carrier for storing and transporting.