Sift flour, baking powder, cinnamon, nutmeg, salt, cloves and ginger together in a large bowl.
Beat caster sugar, butter and brown sugar together in another bowl using an electric mixer until light and crumbly. Add eggs; beat for 1 minute. Add pumpkin puree, treacle and vanilla extract; beat until well blended, about 1 minute. Blend in the flour mixture using the mixer or a wooden spoon until dough is sticky and incorporated.
Cover dough with greaseproof paper; refrigerate for 1 hour.
Preheat oven to 180 C / Gas 4. Line 2 baking trays with baking paper.
Dip your hands into a small bowl of water. Shape the dough into two logs that are about 2cm high, 8cm wide and 25cm long. Place each log onto a baking tray; top with pumpkin seeds.
Bake in the preheated oven until bottoms are dry, 20 to 25 minutes. Let cool for 5 minutes. Reduce oven temperature to 140 C / Gas 1.
Transfer logs to a flat work surface. Cut each log into 10 diagonal slices about 1cm thick. Place sliced biscotti back on the baking tray.
Continue baking until light golden brown, about 20 minutes. Reduce oven temperature to 120 C / Gas mark ½. Continue baking until dry, about 20 minutes more. Transfer to wire cooling racks.
For crisper biscotti, lower the oven temp to 90 C / Gas mark ¼; allow biscotti to remain in oven for another 40 minutes. At that time, turn off the oven and let cool inside oven for 1 hour.