Preheat your oven to 350°F (175°C). Grease and flour two 9-inch round cake pans.
In a large mixing bowl, sift together the flour, sugar, cocoa powder, baking soda, baking powder, and salt.
In another bowl, whisk together the buttermilk, vegetable oil, eggs, and vanilla extract until well combined.
Gradually add the wet ingredients to the dry ingredients, mixing until just combined. Do not overmix.
Gently fold in the fresh blackberries.
Divide the batter evenly between the prepared cake pans.
Bake for 25-30 minutes or until a toothpick inserted into the center comes out clean.
Allow the cakes to cool in the pans for 10 minutes, then transfer to wire racks to cool completely.
While the cakes cool, whip the heavy cream with powdered sugar and vanilla extract until soft peaks form.
Once the cakes are completely cool, place one layer on a serving plate. Spread a layer of whipped cream on top, then place the second layer on top.
Frost the top and sides of the cake with the remaining whipped cream. Garnish with extra blackberries.