1. MAKE THE BLUEBERRY JAM: Add the frozen blueberries, sugar, and cornstarch to a medium saucepan. Cook over medium heat, stirring occasionally, for 20–25 minutes until the jam thickens and reduces to about 3/4 cup total volume — aim for a thick, spreadable consistency, not a thin syrup. Transfer to a bowl and let cool completely. Set aside 2 tablespoons of the finished jam for the frosting.

2. BLOOM THE YEAST AND MAKE THE DOUGH: Combine the warm milk, active dry yeast, and 1 teaspoon of sugar in a small bowl. Stir briefly and let sit undisturbed for 10 minutes until the surface turns foamy — this confirms the yeast is active. In the bowl of a stand mixer fitted with the whisk attachment, combine the flour, salt, and sugar. Add the whisked eggs, vanilla, lemon zest, and very softened butter and mix until blended. Pour in the foamy yeast mixture and mix until everything comes together.

3. KNEAD AND FIRST RISE: Switch the stand mixer to the dough hook. Knead on medium speed for 7–10 minutes until the dough pulls away cleanly from the sides of the bowl, forms a smooth ball, and springs back when pressed lightly. If the dough stays very sticky after 5 minutes, add flour 1 tablespoon at a time. Shape the dough into a ball, transfer to a large greased bowl, cover with plastic wrap, and let rise in a warm spot for 1 to 1.5 hours until doubled in size.

4. MAKE THE LEMON FILLING: While the dough rises, stir together the very softened butter, brown sugar, granulated sugar, lemon zest, lemon juice, and salt until a spreadable paste forms. Set aside at room temperature so it stays soft enough to spread when needed.

5. ROLL, FILL, AND CUT: Punch the risen dough down to release the air and turn it out onto a lightly floured surface. Roll the dough into an 18×12-inch rectangle, about 1/4 inch thick. Spread the lemon filling evenly over the entire surface. Spoon the cooled blueberry jam over the lemon filling and spread into an even layer. Starting from the 18-inch edge closest to you, roll the dough tightly into a log. Trim the uneven ends. Cut 12 rolls about 1.5 inches wide using unflavored dental floss — loop it under the log, cross the ends over the top, and pull to cut cleanly without compressing the rolls.

6. SECOND RISE WITH CREAM SOAK: Place the cut rolls in a greased 9×13-inch casserole dish. Pour the room-temperature heavy cream evenly into the gaps between each roll. Cover with lightly greased plastic wrap and let proof in a warm spot for about 1 hour until the rolls double in size and press against each other. Preheat the oven to 350°F about 15 minutes before the rolls finish proofing.

7. BAKE: Bake the proofed rolls for 22–25 minutes until the tops turn a light golden color. Do not overbake — the rolls should look barely golden and the dough at the center of the pan should still feel soft when pressed gently.

8. MAKE THE FROSTING AND SERVE: Beat the very softened butter in a bowl on high speed for 2 minutes until pale and fluffy. Add the cold cream cheese and mix on medium until smooth. Sift in the powdered sugar on low speed. Add the lemon zest and the 2 tablespoons of reserved blueberry jam, then mix on high until the frosting turns fluffy with beautiful purple swirls. Let the baked rolls cool for 5 minutes, then spread or pour the frosting generously over the warm rolls and serve immediately.