1. Preheat oven to 350 degrees and line pans with cupcake liners.
2. Sift cake mix into a small bowl and set aside.
3. In a large bowl, use a whisk to gently combine remaining cake ingredients.
4. Add cake mix to wet ingredients and stir until smooth.
5. Fill cupcake liners 3/4 full and bake for 15-20 minutes, or until an inserted knife comes out clean.
6. Buttercream: Beat butter for 2 minutes. Slowly add powdered sugar until the frosting becomes very thick. Slowly add berry puree a little at a time. Stop once the buttercream becomes the right consistency. If it becomes too loose, add more powdered sugar to thicken.
7. Pipe buttercream onto cooled cupcakes and top with a fresh blackberry.
To make blackberry puree, place berries in a blender or food processor and mix until smooth. Put the puree through a strainer to remove the seeds.