Preheat oven to 350F (175C) and line a large cookie sheet with parchment paper.
In a large mixing bowl, cream together butter and sugars until light and creamy.
Blot the pumpkin: Place the pumpkin puree in a separate bowl and use a wad of paper towels to blot the pumpkin 10 times, until you’ve removed some of the excess water (if you are using a scale, my pumpkin typically weighs 85g after I’ve thoroughly blotted it).
Add the blotted pumpkin puree, egg, and vanilla extract to the butter and sugar and beat until well-combined.
In a separate mixing bowl, whisk together flour, baking powder, pumpkin spice, and salt.
With mixer on low-speed, gradually stir dry ingredients into wet until combined.
Using lightly floured hands, divide the dough into two equal pieces and form each into a loaf that’s approximately 10-12” (27cm) long by 2-3” (6cm) wide. Space loaves at least 4” (10cm) apart (biscotti will spread! If you don’t have enough space on your cookie sheet, use two sheets). If the dough is sticky and difficult to manage, dust your hands with additional flour.
Transfer to 350F (175C) preheated oven and bake for 30-33 minutes or until edges are light golden brown. Remove from the oven and allow pumpkin biscotti to cool completely before proceeding. Don’t turn off the oven as you’ll need to cook the biscotti a second time.
Once biscotti is cooled completely, use a sharp serrated bread knife to slice loaves on a bias into slices approximately 1” (2.5cm) thick. Return biscotti slices to parchment paper lined baking sheet, placing cut-side down, and return to 350F (175C) oven.
Bake for 10 minutes, remove from oven and use tongs to carefully flip biscotti, and then bake for another 10-15 minutes. Biscotti should be slightly golden when finished baking and the centers should not appear moist.
Allow to cool completely before enjoying or drizzling with white chocolate.
White chocolate drizzle: Place white chocolate in a microwave-safe bowl and heat in 20-second increments until smooth and melted. Add pumpkin spice and stir until combined.
Drizzle over cooled biscotti or dip biscotti slices in white chocolate.