Preheat the oven to 350°F (175°C). Grease the cake pans and line the bottoms with parchment paper.

In a large mixing bowl, whisk together the flour, granulated sugar, cocoa powder, baking soda, baking powder, and salt until well combined.

In another bowl, combine the buttermilk, vegetable oil, eggs, and vanilla extract. Mix well.

Gradually add the wet ingredients to the dry ingredients and mix until smooth. Slowly stir in the boiling water until fully incorporated. The batter will be thin.

Gently fold in the mixed berries into the batter.

Divide the batter evenly between the two prepared cake pans. Bake for 25-30 minutes, or until a toothpick inserted into the center comes out clean.

Remove the cakes from the oven and allow them to cool in the pans for 10 minutes. Then, transfer them to cooling racks to cool completely.

While the cakes are cooling, prepare the frosting. In a large bowl, beat together the softened butter and powdered sugar until smooth. Add the heavy cream and vanilla extract, beating until the frosting is light and fluffy.

Once the cakes are cooled, place one layer on a serving plate. Spread a layer of frosting on top, then carefully place the second layer on top of the frosting.

Frost the top and sides of the cake, using a spatula to create a smooth finish. Decorate the top with additional mixed berries.

Slice the cake, serve, and enjoy!