garnish of sour cream, blueberries, and mint leaves (optional)
Add blueberries, water, honey, cinnamon sticks, ginger, and cardamom to a large saucepan over medium heat. Bring mixture to a low boil, stirring occasionally. Cook for 2-3 more minutes at a low boil until berries begin to burst. Remove from heat and remove cinnamon sticks and cardamom pods.
Stir cornstarch into milk. Add the milk and cornstarch to the berry mixture and return to the heat. Cook over medium heat for 3-5 minutes or until the soup thickens slightly. Remove from heat.
Process the soup in two batches in a food processor. Be careful when processing hot liquids. Pass each batch through a fine mesh strainer and discard solids. Let the soup cool a bit more before placing in the refrigerator, loosely covered. Refrigerate for at least 4 hours before proceeding.
Remove soup from the refrigerator and whisk in 1 cup of sour cream. Serve with a garnish of blueberries, mint, and small dollops of sour cream.