Preheat oven to 375° F (190° C).

Line 2 half sheet rimmed baking trays with parchment or silicone baking mats. Set aside.

In a large mixing bowl or the bowl of a stand mixer, mix pumpkin purée, sugars, butter, and egg on medium-high speed until well-combined.

In a separate bowl, whisk together flour, baking powder, salt, and spices. Add to wet ingredients in 3 additions, mixing well after each addition. Dough will be sticky.

With floured hands, transfer dough to lightly floured countertop.

Divide the dough into two equal portions and shape into 10-inch x 2-inch logs.

Place logs on one of the prepared trays, allowing 3-4 inches of space in between.

Bake logs for 25 minutes. Remove from oven and reduce oven temperature to 325° F (162° C).

Cool on trays for 15 minutes.

Transfer logs to a chopping board. With a serrated knife, slice diagonally into 1/2-inch slices.

Arrange slices on both prepared trays, places the slices upright and dividing them equally between the 2 trays. Bake 20 minutes more, or until dry and golden brown around the edges.

Cool completely on trays.

Melt chocolate over a bain marie or double boiler, or in a microwave-safe bowl at medium temperature, stirring frequently.

Dunk biscotti in melted chocolate, drizzle with a spoon, or pipe in zigzags over biscotti cookies. Finish with a sprinkling of Demerara sugar.

Set aside and allow chocolate to set at room temperature.

Layer biscotti in an airtight container between sheets of parchment or wax paper. Store at room temperature for up to 5 days, or freeze for up to 3 months.