Preheat the oven to 350ºF and line a baking sheet with parchment paper.
In the bowl of a stand mixer fitted with the paddle attachment, beat together the butter, sugar, pumpkin pie spice, cinnamon, baking powder and salt until smooth and creamy, about 2 minutes. Add the egg and pumpkin purée and continue beating. Add the flour and beat just until combined. (The dough will be very sticky.)
Transfer the dough onto the prepared baking sheet and divide it in half. Shape each piece into two 10x2 1/2-inch logs then pat the logs into rectangles.
Bake the biscotti for 25 minutes then remove it from the oven and allow it to cool for 5 minutes. Reduce the oven temperature to 325ºF.
Transfer the biscotti to a cutting board and using a sharp knife, cut each rectangle into 12 slices at a diagonal. Return the biscotti to the baking sheet, standing them up on the bottom edge and spacing them about 1 inch apart.
Return the biscotti to the oven and bake for an additional 25 minutes until golden and crispy. (The biscotti will still be soft in the centers.) Turn off the oven, crack open the oven door and let the biscotti cool in the oven. Once they are completely cool, remove the biscotti from the oven.
Melt the chocolate in the microwave or a double boiler until smooth. Drizzle the biscotti with the chocolate then sprinkle them with the chopped pumpkin seeds. Let cool until the chocolate fully hardens. Serve or store in an air-tight container at room temperature.