Step 1: Prepare the Cake Batter

1. Preheat oven to 350°F (175°C). Grease and line two 8-inch round cake pans with parchment paper.
2. In a large bowl, whisk together flour, cocoa powder, sugar, baking powder, baking soda, and salt.
3. In a separate bowl, whisk buttermilk, oil, eggs, and vanilla extract until combined.
4. Gradually add the wet ingredients to the dry ingredients, stirring until just combined.
5. Slowly pour in the hot coffee or water, mixing until the batter is smooth (it will be thin).
6. Divide the batter evenly between the prepared pans.

Step 2: Bake the Cake

1. Bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean.
2. Let the cakes cool in the pans for 10 minutes, then transfer to a wire rack to cool completely.

Step 3: Make the Raspberry Filling

1. In a saucepan, combine raspberries, sugar, and lemon juice over medium heat.
2. Stir occasionally until the raspberries break down.
3. Add the cornstarch mixture, stirring continuously until thickened.
4. Remove from heat and let cool before using.

Step 4: Prepare the Chocolate Ganache

1. Heat the heavy cream in a small saucepan until it begins to simmer (do not boil).
2. Pour the cream over the chopped dark chocolate in a heatproof bowl.
3. Let sit for 2-3 minutes, then whisk until smooth.
4. Stir in butter for added shine.

Step 5: Assemble the Cake

1. Place one cake layer on a serving plate.
2. Spread a layer of raspberry filling evenly over the cake.
3. Place the second cake layer on top.
4. Pour the chocolate ganache over the top, letting it drip down the sides.

Step 6: Decorate & Serve

1. Garnish with fresh raspberries and chocolate shavings.
2. Let the ganache set for 30 minutes before slicing.
3. Serve and enjoy!