1. Preheat oven to 350 degrees and line pans with cupcake liners.
2. Sift cake mix into a small bowl to remove lumps and set aside.
3. In a large bowl, combine eggs, melted butter, milk, sour cream and vanilla extract.
4. Stir in cake mix until smooth. Add chocolate chips and stir again.
5. Fill cupcake liners 3/4 full and bake for 17-22 minute or until an inserted knife comes out clean.
6. Buttercream: Beat butter for 2 minutes. Add cocoa powder, vanilla extract and milk and beat again. Slowly add powdered sugar until you reach your desired consistency.
7. Pipe onto cooled cupcakes and finish with extra mini chocolate chips.