Preheat your oven to 350°F (175°C) and line a muffin tin with cupcake liners.
In a large mixing bowl, whisk together the flour, cocoa powder, sugar, baking powder, baking soda, and salt until well combined.
Add the softened butter, eggs, vanilla extract, and buttermilk to the dry ingredients. Beat with an electric mixer on medium speed for about 2 minutes until smooth and creamy.
Gently fold in the fresh blackberries until evenly distributed throughout the batter.
Divide the batter evenly among the prepared cupcake liners, filling each about 2/3 full.
Bake in the preheated oven for 18-20 minutes, or until a toothpick inserted into the center comes out clean.
Remove from the oven and let the cupcakes cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.
While the cupcakes are cooling, prepare the frosting. In a mixing bowl, beat the softened butter until creamy.
Gradually add the powdered sugar and cocoa powder, mixing until combined.
Add the heavy cream and vanilla extract, then beat on high speed for about 2 minutes until the frosting is light and fluffy.
Once the cupcakes are completely cool, frost them with the chocolate frosting using a piping bag or a spatula. Top with additional blackberries if desired.