In the bowl of a countertop mixer fitted with the paddle attachment, add the butter, sugar, brown sugar, pumpkin pie spice, salt, vanilla, and baking powder. Mix on medium-low speed until mixture is fully combined and smooth.

Add the egg, egg yolk and pumpkin puree; mix on low speed until fully absorbed.

Add the flour and mix on low speed until fully combined.

Transfer dough to a lined baking sheet and shape into a 5"x10" log. (Note: The dough will be very sticky due to the pumpkin. Just wet your hands slightly to keep the dough from sticking to your hands at this stage.)

Sprinkle a generous amount of turbinado or sanding sugar across the top of the log. (optional)

Bake at 350°F for 28 minutes.

Remove pan from oven and allow it to cool for 10 minutes. Transfer biscotti to a cutting board and use a sharp knife to slice the log crosswise into ¾" slices. Place each biscotti cut-side down on a lined-baking sheet.

Reduce oven temperature to 325°F and continue baking for another 38-40 minutes, or until the biscotti turn light golden brown. (Tip: Press the center of the biscotti slightly with your finger. If they still feel soft, then continue baking them until they turn slightly crispy.)