1. Preheat your oven to 350°F (175°C) and line a muffin tin with cupcake liners.
2. In a medium bowl, whisk together the flour, cocoa powder, baking powder, baking soda, and salt.
3. In a large bowl, whisk together the sugar, vegetable oil, egg, and vanilla extract until well combined.
4. Add the dry ingredients to the wet ingredients, alternating with the buttermilk, beginning and ending with the dry ingredients. Mix until just combined.
5. Divide the batter evenly among the cupcake liners, filling each about two-thirds full.
6. Bake for 15-18 minutes, or until a toothpick inserted into the center comes out clean. Allow the cupcakes to cool completely before frosting.

For the Blackberry Buttercream:

1. Puree the fresh blackberries in a food processor or blender. Strain the puree through a fine mesh sieve to remove the seeds.
2. In a large bowl, beat the softened butter until creamy. Gradually add the powdered sugar, one cup at a time, beating well after each addition.
3. Add the vanilla extract, pinch of salt, and blackberry puree to the butter mixture. Beat until the buttercream is light and fluffy.
4. Pipe or spread the blackberry buttercream onto the cooled cupcakes. Garnish with fresh blackberries if desired.