Whisk the cornstarch and water together until all the cornstarch has dissolved. Combine cornstarch mixture, raspberries, 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. No need to cover it, but if refrigerating for longer than 4 hours, cover tightly. It will continue to thicken up as it chills.

Preheat oven to 350°F (177°C). Grease three 9-inch cake pans, line with parchment paper rounds, then grease the parchment paper.

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, 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.

Divide batter evenly between 3 pans. Bake for approximately 24–26 minutes. 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.

Place 1 cooled cake layer on your cake stand or serving plate. Cover the top with about ¼ cup of chocolate buttercream frosting. Pipe a thick border of buttercream around the edge. Spread half of the thickened and chilled raspberry filling inside the buttercream border. Place second cake layer on top and repeat the filling process: spread frosting, pipe border, then spread on remaining raspberry filling. Top with third cake layer.

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

Make the ganache by heating cream and liqueur in a small saucepan, then pouring over the chopped chocolate. Stir until smooth and let cool.

Pour/spoon thickened ganache on chilled cake. Spread all over the cake. Garnish with fresh raspberries, if desired.

Cover leftover cake tightly and store in the refrigerator for 5 days.