Add the blueberries, granulated sugar, cornstarch, and lemon juice to a saucepan over low heat. Stir continuously while the blueberries break down and release their juices. Turn the stove to medium heat, and allow the mixture to boil for about 5 minutes until the sauce thickens. Remove from the heat and transfer to a bowl. Let the blueberry sauce cool in the fridge until ready to use.
PREPARE THE DOUGH:
In a small bowl, combine the warm water, 1 teaspoon of sugar, and yeast. Let it sit for about 5 to 10 minutes until a foam forms on top. In a separate large mixing bowl, whisk together the flour, sugar, and salt. Add the softened butter, eggs, vanilla, warm milk, and the yeast mixture. Mix until a dough forms. Cover with plastic wrap and set in a warm place to rise for about 45 minutes.
Punch down the dough, and transfer to a floured surface.Roll the dough out into a rectangle about 15x20 inches.Spread the filling on top of the dough. Roll into a log.
Butter a 9x13 inch baking pan. Using a string of floss to wrap around the dough and pull to cut. Cut the dough into 12 equal rolls and place in the prepared baking pan.Cover and let rise in a warm place for about 30 minutes or until the rolls double in size. While the rolls rise, preheat the oven to 350°F.
Bake for 25 to 30 minutes or until the rolls become golden. Remove from the oven and let them cool while making the frosting.
ADD THE FROSTING:
In a medium mixing bowl, use an electric mixer to combine the butter, cream cheese, and vanilla extract until smooth. Gradually add the powdered sugar until completely combined. Frost the rolls while still warm.