Step 1: Prepare the Blueberry Filling

1. In a medium saucepan over medium-high heat, combine the blueberries, brown sugar, and lemon juice. Stir occasionally and let it cook for about 20 minutes, until the berries break down and thicken.
2. In a small bowl, dissolve the cornstarch in the water. Stir this mixture into the blueberries.
3. Continue cooking for an additional 2 minutes, stirring constantly, until the filling thickens.
4. Remove from heat and set aside 1-2 tablespoons of the jam for the frosting. Transfer the remaining filling to a bowl and allow it to cool completely.

Step 2: Make the Tangzhong

1. In a small saucepan, whisk together the water and all-purpose flour.
2. Cook over medium heat, whisking continuously, for about 4-5 minutes, until the mixture thickens into a smooth paste.
3. Remove from heat and transfer it to a small bowl. Let it cool before using.

Step 3: Prepare the Dough

1. In a large mixing bowl, combine the all-purpose flour, granulated sugar, yeast, and salt.
2. Add the egg, milk, vanilla extract, and the prepared Tangzhong. Mix until a rough dough forms.
3. Using a stand mixer with a dough hook attachment or kneading by hand, begin incorporating the softened butter, one piece at a time, ensuring each piece is fully absorbed before adding the next.
4. Knead the dough for 8-10 minutes until it becomes smooth and elastic. The dough should stick slightly to the bottom of the bowl but not the sides. If it is too wet, add flour one tablespoon at a time. If too dry, add a small amount of milk.
5. Lightly grease a large bowl with butter or oil. Shape the dough into a smooth ball and place it in the bowl, seam-side down. Cover with plastic wrap and let it rise in a warm place for about 1 hour, or until doubled in size.

Step 4: Roll Out the Dough and Add the Filling

1. Lightly flour a clean work surface and roll out the dough into a 20×15-inch rectangle, with the long side facing you.
2. In a small bowl, mix together the dark brown sugar, butter, and cinnamon until smooth.
3. Using an offset spatula, spread the cinnamon mixture evenly over the rolled-out dough, leaving a ½-inch clean border along the top edge.
4. Spread the cooled blueberry filling evenly over the cinnamon layer.

Step 5: Shape the Rolls

1. Starting from the long edge closest to you, carefully roll the dough into a tight log.
2. Pinch the seam to seal in the filling.
3. Using a sharp knife or unflavored dental floss, cut the log into 12 equal rolls, each about 1 ½ inches thick.
4. Line a 9×13-inch baking pan with parchment paper and arrange the rolls inside.
5. Cover the pan with plastic wrap and let the rolls rise for another 45-60 minutes, or until doubled in size.

Step 6: Bake the Rolls

1. Preheat the oven to 325°F (162°C).
2. Bake the rolls for 23-26 minutes, until the tops are lightly golden brown.
3. Remove the pan from the oven and let the rolls cool slightly while preparing the frosting.

Step 7: Make the Blueberry Cream Cheese Frosting

1. In a medium bowl, mix together the butter, cream cheese, and powdered sugar until smooth.
2. Whisk in the heavy cream and reserved blueberry jam until well combined.
3. Spread the frosting over the warm cinnamon rolls before serving.