Combine the stock powder with 1 litre of boiling water.
Heat oil in a large saucepan, add onion, garlic and cumin and cook over medium-low heat for about 10 minutes, until onion is soft. Add cumin, cook, stirring for a minute. Add pumpkin, chipotle and stock, bring to a simmer over medium to high heat and cook uncovered for about 10 minutes, or until pumpkin is soft.
Deep fry tortillas one at a time in hot vegetable oil (about 180C) until golden and crisp. Drain on absorbent paper; break into large pieces.
Serve the soup with a spoonful of sour cream and garnish with orange zest, coriander, freshly ground black pepper and tortilla crisps.
* Chipotle in adobo is available from most delicatessens in the Mexican section.