In a stand mixer, combine active sourdough starter, warm milk, 50g sugar, and eggs. Add bread flour and salt. Mix using a dough hook until shaggy.
Gradually add 85g of softened butter, one tablespoon at a time. Knead on medium speed for 10-15 minutes until it passes the windowpane test.
Transfer to an oiled bowl, cover, and let ferment at room temperature for 4-6 hours until increased by 50%.
Place the covered bowl in the refrigerator overnight (8-12 hours) for the cold retard.
Simmer blueberries, 60g sugar, cornstarch, lemon juice, and cardamom in a saucepan for 10 minutes until thick. Let cool completely.
Roll the cold dough out into a 12x18-inch rectangle. Spread the cooled blueberry filling evenly, leaving a 1/2-inch border.
Roll into a tight log and cut into 12 even slices. Place in a parchment-lined 9x13-inch baking dish.
Cover and let proof in a warm spot for 3-5 hours until puffy and touching.
Bake at 375°F (190°C) for 30-35 minutes until golden brown.
Beat cream cheese, remaining 30g butter, powdered sugar, and vanilla until smooth. Spread over the warm rolls and serve.
