Preheat your oven to 350°F (175°C).

Prepare a 12-cup muffin tin by lining it with cupcake liners.

In a medium mixing bowl, combine flour, cocoa powder, baking powder, baking soda, and salt. Whisk until well combined.

In a large mixing bowl, whisk together sugar and eggs until light and fluffy.

Add sour cream and vegetable oil to the egg mixture and mix well.

Gradually add the dry ingredients to the wet mixture, stirring gently until just combined.

Fold in blackberries and dark chocolate chips gently.

Fill each cupcake liner about two-thirds full with the batter.

Bake for 18-20 minutes, or until a toothpick comes out clean.

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

For frosting, mix powdered sugar with a small amount of water until pourable, then drizzle over cooled cupcakes.

Garnish with extra blackberries before serving.