Preheat the oven to 350°F (175°C) and line a muffin tin with cupcake liners.

In a mixing bowl, whisk together the flour, cocoa powder, sugar, baking powder, baking soda, and salt.

In another bowl, combine the melted butter, eggs, vanilla extract, and buttermilk. Mix well.

Gradually add the wet ingredients to the dry ingredients, mixing until just combined.

Fold in the blackberries gently.

Divide the batter evenly among the cupcake liners.

Bake in the preheated oven for 20-25 minutes, or until a toothpick inserted into the center comes out clean.

Allow the cupcakes to cool in the tin for 5 minutes, then transfer to a wire rack to cool completely.

In a small saucepan, heat the heavy cream over medium heat until it begins to simmer.

Remove from heat and add the dark chocolate chips. Let sit for 5 minutes, then stir until smooth.

Once the cupcakes are completely cool, drizzle the chocolate ganache over the tops.

Garnish with additional blackberries if desired.