Step 1: Bake the Chocolate Cake

1. Preheat oven to 350°F (175°C). Grease and flour two 9-inch round cake pans and line with parchment paper.
2. In a large mixing bowl, whisk together sugar, flour, cocoa powder, baking powder, baking soda, and salt.
3. Add eggs, buttermilk, oil, and vanilla extract. Mix until well combined.
4. Stir in boiling water (the batter will be thin—this is normal).
5. Divide batter evenly between prepared pans and bake for 30-35 minutes, or until a toothpick inserted in the center comes out clean.
6. Let cakes cool in pans for 10 minutes, then transfer to a wire rack to cool completely.

Step 2: Make the Raspberry Filling

1. In a small saucepan, combine raspberries, sugar, and lemon juice over medium heat. Stir occasionally until raspberries release their juices.
2. In a small bowl, mix cornstarch with water to make a slurry, then stir it into the raspberry mixture.
3. Simmer for about 5 minutes, stirring frequently, until the mixture thickens.
4. Remove from heat and let it cool completely before using.

Step 3: Prepare the Chocolate Buttercream

1. In a large bowl, beat softened butter until creamy.
2. Add cocoa powder, powdered sugar, heavy cream, vanilla, and salt. Beat on low speed at first, then increase to medium-high and whip until smooth and fluffy.
3. If the frosting is too thick, add more heavy cream; if too thin, add more powdered sugar.

Step 4: Assemble the Cake

1. Place one cooled cake layer on a serving plate. Spread a layer of raspberry filling on top.
2. Add the second cake layer. Spread a thin crumb coat of chocolate buttercream over the entire cake and chill for 15 minutes.
3. Frost the cake with the remaining buttercream. Optionally, top with fresh raspberries and drizzle with chocolate ganache.