Preheat oven to 325 and fill muffin tins with cupcake liners.
Sift cake mix into a medium bowl. In a larger bowl combine, greek yogurt (or sour cream), oil, buttermilk, vanilla, and eggs and whisk until smooth. Add cake mix and mix until smooth.
Fill cupcake liners 3/4 full and bake 13-18 minutes until an inserted toothpick comes out clean.
After cupcakes have cooled completely, cream the butter for two minutes in a large bowl. Add vanilla and heavy cream and mix until smooth. Mix in powdered sugar one cup at a time until desired consistency is reached (**if piping the frosting, double the frosting recipe and look for a slightly stiffer consistency) Spread or pipe frosting onto cupcakes. Store in airtight container at room temperature.
Store in airtight container at room temperature up to three days, or in the fridge up to five days.