Combine 12 oz frozen blueberries, 1/3 cup granulated sugar, and 1 tsp cornstarch in a medium saucepan. Cook over medium heat, stirring occasionally, until the blueberries release juice and the mixture comes to a simmer (about 6–8 minutes).

Reduce heat and simmer gently until slightly thickened, about 5–7 more minutes. Taste and adjust with a tablespoon more sugar if needed for sweetness.

Remove from heat, let cool to room temperature. The jam will thicken as it cools. Spoon 2 tbsp aside for the frosting later.

Warm 1 1/4 cups whole milk to about 110°F. Sprinkle 2 1/4 tsp active dry yeast and 1 tsp sugar over the milk. Let sit 5–10 minutes until foamy.

In the bowl of a stand mixer, whisk together 4 3/4 cups all-purpose flour, 1 1/2 tsp salt, and 2 tbsp sugar.

Add the foamy yeast-milk, 2 whisked eggs, 1/2 cup very softened butter, 1/2 tbsp vanilla, and 2 tbsp lemon zest. Mix with the dough hook on low until a shaggy dough forms, then increase speed to medium and knead 5–7 minutes until smooth and elastic.

If dough is too sticky, add flour 1 tbsp at a time; if too stiff, add 1 tsp water.

Transfer dough to a lightly greased bowl, cover with plastic wrap or a towel, and let rise in a warm place until doubled, about 1–1.5 hours.

In a small bowl, cream together 1/2 cup softened butter, 1/2 cup brown sugar, 1/2 cup granulated sugar, 2 tbsp lemon zest, 2 tbsp lemon juice, and 1/4 tsp salt until smooth and fluffy.

Taste and adjust with more lemon juice or zest if desired.

Turn the risen dough onto a lightly floured surface. Roll into a large rectangle about 16x12 inches (or similar — aim for about 1/4-inch thickness).

Spread the lemon filling evenly over the dough, leaving a 1/2-inch border on one long side.

Spoon dollops of the cooled blueberry jam across the filling, press jam into filling so it won’t leak during rolling.

Roll tightly from the long edge into a log. Slice into 12 even rolls (about 1 1/2 inches thick).

Place rolls cut-side up in a greased 9x13-inch pan, leaving a little room to expand. Cover and let rise 30–45 minutes until puffy.

Preheat oven to 350°F. Right before baking, drizzle 1/4 cup heavy cream evenly between the rolls.

Bake 25–30 minutes until golden on top and cooked through.

Beat 6 tbsp softened butter and 6 oz cold cream cheese until smooth.

Add 3/4 cup powdered sugar, 1/2 tbsp lemon zest, and reserved 2 tbsp blueberry jam. Beat until fluffy and spreadable.

Spread or drizzle frosting over warm rolls before serving.