Place all ingredients, except the soy milk powder, into a large saucepan. Boil until pumpkin is tender. Spoon into a food processor, add soy milk powder, and process until smooth and creamy. Adjust seasoning if necessary.
(*This volume will need to be done in 3-4 separate lots. Just place each lot into a bowl when processed and stir when completed.)
Serve with a dollop of vegan sour cream, if desired.