Preheat your oven to 325°F, add the blueberries, juice, and the 3 tablespoons of brown sugar to a pan over medium-high heat, and then let them cook for 13 minutes. Combine the corn starch and the 2 tablespoons of water, add the mixture to the blueberry mixture, and then let them cook for 2 minutes. Reserve 4 tablespoons, transfer the remaining mixture to a bowl, and then add the ¼ cup of flour and remaining water to a pan over medium heat. Whisk them together for 5 minutes, transfer them to a bowl, and then combine the yeast, sugar, salt, and remaining flour. Add the milk, cream, egg, and water mixture, knead in 1 tablespoon of the butter, and then repeat this process 7 more times. Knead the mixture for 12 minutes, combine the cinnamon, remaining brown sugar, and ½ cup of the butter, and then add the dough to a floured surface. Roll it into a 20 x 16 inch rectangle, add the cinnamon mixture, and then add the bowl of blueberry mixture. Cut it into 12 strips, roll them up, and then add them to a 9 x 13 inch parchment-lined baking pan. Cover them with plastic wrap, let them rise for 1 hour, and then let them bake for 40 minutes. Beat together the mascarpone, powdered sugar, 2 tablespoons of the blueberry mixture, and the remaining butter on low speed, let the rolls cool for 15 minutes, and then divide the mascarpone mixture and remaining blueberry mixture between them.