If you are cooking pumpkin anyways, take out the seeds and roast them in the oven with a bit of salt until crispy.
Let them cool down
Blend the seeds
Add 1/2 to 1 liter of water and Blend. Let the mixture stay for half a day or over night. Blend. I used all seeds from 1 hokaido.
Use a cloth or a strainer to separate pulp from milk.
If you like you can add vanilla, cinnamon, cardamon, ginger or whatever taste you like.