Chocolate Raspberry Cake

Instructions

Make the raspberry filling: 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.)
Make the cake: 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.
As the cakes cool, make the chocolate buttercream: 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.
Begin layering with raspberry filling and buttercream: 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,