Combine the blueberries, 3 tablespoons brown sugar, and lemon juice in a saucepan over medium heat. Simmer for 20 minutes, stirring occasionally, until the blueberries break down and release their juices. In a small bowl, whisk together the cornstarch and 2 tablespoons water until smooth, then stir this mixture into the simmering blueberries. Cook for another 2 minutes until the filling thickens slightly. Remove from heat and let cool for a few minutes, then transfer to a bowl and reserve 2 tablespoons of the filling for the frosting later. Set the remaining filling aside to cool completely.

In a small saucepan, whisk together 2/3 cup water and 1/4 cup flour over medium heat, stirring constantly for 4-5 minutes until the mixture thickens into a smooth paste that pulls away from the sides of the pan slightly. This tangzhong base adds moisture and tenderness to the dough. Remove from heat and let cool for a few minutes before using.

In a large mixing bowl, combine 3 2/3 cups all-purpose flour, 1/3 cup sugar, 2 1/4 teaspoons yeast, and 1 teaspoon salt. Add the lukewarm milk, heavy cream, egg, and the cooled tangzhong paste from Step 2. Mix on low speed until a shaggy dough forms, then knead for 2 minutes. Add the 8 tablespoons softened butter and 1 teaspoon vanilla extract, then continue kneading for 12 minutes until the dough becomes smooth, elastic, and slightly sticky. I like to knead a bit longer if the dough feels too sticky—it develops better gluten structure and makes for fluffier rolls.

Transfer the dough from Step 3 to a lightly oiled bowl, cover with a damp towel, and let rise in a warm place for 1-2 hours until doubled in size. While the dough rises, prepare the cinnamon-sugar filling by mixing 1/2 cup softened butter, 3/4 cup brown sugar, and 1 tablespoon ground cinnamon in a bowl until well combined. Once the dough has risen, turn it out onto a lightly floured surface and roll it into a 21x15 inch rectangle. Spread the cinnamon-sugar mixture evenly over the dough, then spread or drizzle the cooled blueberry filling from Step 1 over the top.

Starting from the long side of the rectangle, tightly roll the dough into a log, then cut it into 12 equal pieces using a sharp knife or dental floss (I find floss creates the cleanest cuts without squishing the rolls). Place the rolls cut-side up in a greased 9x13 inch baking dish, spacing them slightly apart. Cover with a damp towel and let rise for 1-2 hours until they're puffy and touching each other.

Preheat your oven to 325°F. Once the rolls have finished their second rise, bake them for 30-40 minutes until golden brown on top and the internal temperature of the center roll reaches 185°F when measured with an instant-read thermometer. If the tops start browning too quickly, tent them loosely with foil for the last 10-15 minutes.

While the rolls bake, prepare the cream cheese frosting by beating together 4 ounces softened cream cheese, 6 tablespoons softened butter, 1 1/4 cups icing sugar, the reserved 2 tablespoons blueberry filling from Step 1, 1 1/2 tablespoons heavy cream, and a pinch of fine sea salt until smooth and spreadable. Remove the baked rolls from the oven and let them cool for 15 minutes in the pan. Spread the frosting generously over the warm rolls. If you have any additional blueberry filling remaining, you can swirl it into the frosting for a more marbled appearance and extra blueberry flavor.