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. Stir the mixture, mashing the raspberries as they begin to thaw and soften. Bring to a boil and let it boil for 5 minutes, stirring occasionally. Remove pan from heat and stir in vanilla extract.

Allow cooling for 10–15 minutes, then transfer to a bowl or container and refrigerate for at least 4 hours.

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 together in a large bowl. Set aside.

Mix the oil, eggs, and sour cream together on medium-high speed until combined. Add the buttermilk and vanilla and beat until combined.

Pour wet ingredients into dry ingredients, add the hot water or coffee, and whisk or beat on low speed until completely combined. Fold in flour-coated chocolate chips.

Divide batter evenly between 3 pans. Bake for 24–26 minutes. The cakes are done when a toothpick inserted in the center comes out clean.

Allow to cool completely in the pan.

Beat 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 for 1 minute.

Place 1 cooled cake layer on your cake stand or serving plate. Cover the top with about ¼ cup of chocolate buttercream. Pipe a thick border of buttercream around the edge.

Spread about ½ cup of thickened raspberry filling inside the buttercream border. Place the second cake layer on top and repeat the filling process.

Top with the third cake layer.

Using the remaining chocolate buttercream, spread a thin layer for a crumb coat. Chill uncovered in the refrigerator for at least 30 minutes.

Make the chocolate ganache by placing finely chopped chocolate in a medium heat-proof bowl. Heat the cream and raspberry liqueur, then pour over chocolate. Let sit for 2–3 minutes to gently soften the chocolate.

Stir until smooth and let it chill for 30 minutes before spreading on the chilled cake.

Pour/spoon ganache on chilled cake, and spread all over with an icing spatula. Garnish with fresh raspberries if desired.