• Preheat your oven to 350°F (175°C). Grease and flour three 8-inch round cake pans.
• In a medium bowl, whisk together the flour, cocoa powder, baking powder, baking soda, and salt. Set aside.
• In a large mixing bowl, cream together the softened butter and granulated sugar until light and fluffy.
• Beat in the eggs, one at a time, followed by the vanilla extract.
• Gradually add the dry ingredients to the wet ingredients, alternating with the buttermilk, beginning and ending with the dry ingredients. Mix until just combined.
• Divide the batter evenly among the prepared cake pans and smooth the tops with a spatula.
• Bake in the preheated oven for 25-30 minutes, or until a toothpick inserted into the center of the cakes comes out clean.
• Allow the cakes to cool in the pans for 10 minutes before transferring them to a wire rack to cool completely.

For the raspberry filling:

• In a saucepan, combine the fresh raspberries, granulated sugar, water, and cornstarch.
• Cook over medium heat, stirring occasionally, until the raspberries break down and the mixture thickens, about 5-7 minutes.
• Remove from heat and let the raspberry filling cool completely before using.

For the frosting:

• In a large mixing bowl, beat the softened butter until creamy.
• Gradually add the powdered sugar and cocoa powder, beating well after each addition.
• Add the vanilla extract and 4 tablespoons of heavy cream, and beat until smooth and creamy. If the frosting is too thick, add more heavy cream, one tablespoon at a time, until desired consistency is reached.

Assembly:

• Once the cakes are completely cooled, place one layer on a serving plate or cake stand.
• Spread a layer of chocolate frosting over the top of the cake layer.
• Spoon a layer of raspberry filling over the frosting.
• Repeat the process with the remaining cake layers, frosting, and raspberry filling, ending with a layer of frosting on top.
• Use a spatula to create a rustic naked look by lightly spreading the frosting around the sides of the cake, leaving some of the cake layers exposed.
• Decorate the top of the cake with fresh raspberries, chocolate shavings or curls, and fresh mint leaves for a beautiful finishing touch.