This recipe is amazing and really simple to get right. However, a word of caution for the frosting; although delicious, it's a bit runny so be prepared to get your hands dirty.
Preheat oven to 350 F.
In a medium bowl combine sugar, buttermilk, oil, eggs, pumpkin, salt, and vanilla.
In another medium bowl, whisk together flour, cocoa powder, baking soda, and baking powder. Gradually add to wet mixture (about 1/2 cup at a time). Beat until smooth.
Line muffin pans with cups and fill each cup with about 3 tbsp of batter.
Bake for about 25 minutes, and allow muffins to cool completely before frosting.
In a small bowl, combine mascarpone, pumpkin pie spice, pumpkin, vanilla, and salt until smooth. Add powdered sugar. Chill in the freezer for about an hour, and spread on cupcakes right before serving.