Heat oven to 350 degrees F.
Line a sheet pan with parchment paper. Set aside.
In a medium bowl, combine flour, pumpkin pie spice, baking soda, and salt. Set aside.
With an electric mixer, beat butter and sugar.
Mix in eggs and vanilla.
Beat in pumpkin.
Stir in flour mixture.
Form into a log.
Place on prepared pan.
Flatten log to 1 inch thick.
Bake for 35 minutes, or until firm.
Remove from oven.
Reduce oven temperature to 250 degrees F.
Using a serrated knife, slice ends off biscotti.
Slice biscotti horizontally into 1 inch slices.
Place slices back on pan.
Bake for an additional 25 minutes per side, until crisp.
Remove cookies to cooling rack.
Dip each cooled cookie into melted chocolate and sprinkle with additional chopped walnuts, if desired.