1. Preheat your oven to 350°F (175°C) and grease and line two 8 or 9-inch round baking pans to ensure easy removal of the cake layers.
2. In a large mixing bowl, whisk together dry ingredients: flour, granulated sugar, cocoa powder, baking soda, baking powder, and salt.
3. Add eggs, buttermilk (or milk), oil, and vanilla extract to the dry mixture. Mix well at medium speed for about 2 minutes until fully combined.
4. Slowly pour in boiling water or hot coffee into the batter while mixing. This enhances the chocolate flavor and creates a smooth batter.
5. Divide the batter evenly between the prepared baking pans and bake for 25-35 minutes. Test doneness with a toothpick; it should come out clean.
6. Let the cakes cool in the pans for 10-15 minutes before carefully transferring them to a wire cooling rack. Allow them to cool completely to prevent breaking.
7. Prepare the raspberry filling by cooking granulated sugar, cornstarch, water, and lemon juice in a saucepan over medium heat. Add raspberries and stir until the mixture thickens. Strain for a seedless filling if desired.
8. Let the raspberry filling cool completely before assembling the cake.
9. Place one cake layer on a serving plate or cake stand. Pipe a border of chocolate buttercream around the edge to create a border for the raspberry filling.
10. Spread the cooled raspberry filling evenly within the chocolate buttercream border.
11. Place the second cake layer on top, aligning it carefully. Repeat the filling and layering process if there are additional layers.
12. Frost the entire cake with chocolate frosting, smoothing it out for a clean finish.
13. Optionally, decorate the top and sides with chocolate ganache, fresh raspberries, or chocolate shavings for an elegant look.
14. Allow the cake to set for a few hours before cutting to ensure all layers hold together perfectly.
15. Serve at room temperature and enjoy the rich combination of chocolate and raspberries with your guests.