Cut kabocha in half, scoop out seeds and membranes, and set aside.
Prick the kabocha flesh with a fork, brush with olive oil and place face down on a baking dish. Bake for about 40 mins at 350F/175°C or until tender.
While the kabocha is roasting, clean and dry the seeds (as best you can) and toast them in a dry pan on med-high heat for about 5 minutes or until nicely golden brown and set aside. Alternately, you can bake them in the oven for about 10-20 minutes at 400F / 200°C.
In a large pot, saute sliced onion in butter until soft.
When the kabocha is soft, carefully (it's hot!) scoop the flesh in chunks out of the skin and add to the pot with onions. Alternately, you can wait for it too cool or roast it already peeled.
Add the bouillon powder, water and cinnamon, and simmer for about 20 minutes. Cover the pot to keep it from splattering.
Pour the mixture into a large bowl. In batches, puree the mixture in a blender (only fill about 3/4 or less full), and add it back to the pan. If you have an immersion blender, even better, and you can just puree it all in the pan.
Stir in the milk and heat gently over low heat, stirring frequently. Mix in salt and pepper to taste, ladle into bowls and top with toasted seeds.