Preheat oven to 350. Line 26 cupcake cups with paper liners.
Beat the cake mix with the eggs, buttermilk and oil until blended. Divide the batter evenly between the prepared cupcake pans.
Bake for 15-20 minutes. Remove from the oven, transfer to a wire rack, and let cool completely.
Tint the vanilla frosting with the orange food coloring. Line the cupcakes snugly into a large pumpkin shape on a large plate or a piece of cardboard covered in foil. Follow this pattern-one row of 4 cupcakes, one row of 5, one row of 6, another of 5, another of 4. This leaves 2 cupcakes to be placed at the pumpkin's top for a stem.
Spread orange frosting over the cupcakes, covering them completely so they look like one big cake. Use black M&M's to outline the pumpkin, and to make the eyes, mouth and nose. Fill in the remaining area of the pumpkin with orange M&M's.