Make cupcakes: Preheat oven to 350°. In a medium bowl, with a mixer on medium speed, beat sugar, buttermilk, oil, eggs, pumpkin, salt, and vanilla until well blended.
Whisk together flour, cocoa, baking soda, and baking powder in a medium bowl. Gradually add to wet mixture, beating on medium speed until smooth.
Line muffin pans with muffin cups and fill each cup with 3 tbsp. batter. Bake until a toothpick inserted in centers comes out clean, 25 minutes. Cool completely.
Make frosting: In a medium bowl, beat mascarpone, pumpkin pie spice, pumpkin, vanilla, and salt with a mixer until smooth. Beat in powdered sugar. Chill until firm, 1 hour. Spread on cupcakes and top each with whipped cream and a pastille. Chill until frosting is firm, about 30 minutes.
*Find chocolate pastilles in the baking aisle at well-stocked grocery stores.
Note: Nutritional analysis is per cupcake.