Preheat the oven to 350ºF (180ºC). Line a baking sheet with parchment paper and set aside.
Whisk the flour, pumpkin pie spice, baking powder, and salt together in a medium bowl, then set aside.
In a second bowl, beat the pumpkin purée, butter, and both sugars with an electric mixer on medium-high speed until well combined, about 2 minutes. Scrape down the sides of the bowl, then add the eggs and vanilla and mix again until combined.
Switch to a rubber spatula or wooden spoon and gently fold the flour mixture into the wet ingredients until just combined.
Scoop half of the biscotti batter into a long row down one side of the baking sheet and repeat on the other. Lightly wet your hands and smooth the tops and sides into flat logs, adding more water as needed. Sprinkle the tops with coarse sugar if using.
Bake the logs for 25 minutes. Remove from the oven and let cool for about 10 minutes.
Using a serrated knife, cut the logs into slices about ½ to 1 inch (1.25 to 2.5 cm) wide. Arrange the slices on their sides on the baking sheet. Bake for 22 to 25 minutes, or until golden and crisp. Let cool completely before storing.
Notes
For detailed ingredient notes, substitution tips, variations, and step-by-step photos, scroll through the full recipe post.
Use a digital kitchen scale for precise measurements and an oven thermometer to confirm your oven is heating correctly.
Shape the logs quickly once the dough is mixed, as the batter is sticky and easier to manage when handled with damp hands.
Let the baked logs cool for at least 10 minutes before slicing so they hold their shape and crumble less.
Storage
Store cooled biscotti in an airtight container at room temperature for up to two weeks. For longer storage, freeze in a sealed container or freezer bag for up to three months and let thaw at room temperature for a few minutes before serving.