In a large mixing bowl, combine the sugar, oil, pumpkin and eggs.
In another bowl, whisk together the flour, baking soda, salt, baking powder and cinnamon; add to pumpkin mixture and beat until well mixed.
Fill paper-lined muffin cups two-thirds full. Bake at 350° for 18-22 minutes or until a toothpick comes out clean. Cool for 10 minutes before removing from pans to wire racks to cool completely.
To make the frosting, place the butter and Biscoff spread into a large bowl. Beat with a mixer until well blended.
Add half the powdered sugar, half the milk, and the Tbsp vanilla to the butter mixture. Mix until blended.
Add the remaining sugar and milk. Mix until blended and the beat with mixer for 3 minutes until nice and fluffy. Add more milk if needed to received desired consistency.
Pipe frosting onto the cooled cupcakes and sprinkle with crushed Biscoff cookies if desired.