Preheat oven to 350 degrees F. Line 2 muffin tins with about 18 paper liners. Set aside.
In a large bowl, combine the cake mix, dry pudding mix, buttermilk, oil, eggs and vanilla. Beat well with an electric mixer for about 2 minutes or until combined and fluffy.
Distribute batter evenly among baking cups, filling about 3/4 full. Bake for approx. 15-17 minutes or until a toothpick inserted near the center comes out clean or with moist, not wet, crumbs. Cool completely.
To make the frosting: In the bowl of a stand mixer, cream together the butter, cocoa powder and vanilla until creamy, about 1 minute. Add in the powdered sugar, one cup at a time, beating well after each addition. Stream in the heavy cream a little at a time to thin out the frosting if it is too thick. Frosting should be light and fluffy.
Pipe the frosting onto the cooled cupcakes; garnish with chocolate sprinkles.