Peel and cut the pumpkin in cubes. Steam for about 25 minutes (stick a knife into a pumpkin cube to check for doneness. The pumpkin will be soft when cooked). Blend to puree.
Preheat your oven to 200C (392F). Grease a loaf pan.
Whisk butter and sugar. Add eggs, one by one. Sift flour and baking powder together and add to butter and sugar mixture. Add the milk.
Divide the dough in 2. In one, add the pumpkin and the pumpkin spice, in the other add the cacao plus 20g milk.
Alternate the layers of dough in the loaf pan to create a marble effect. Add pepitas on top of the cake before baking 40 to 50 minutes. Lower temperature to 180 C (356 F) after 10 minutes.