In a microwave-safe container, combine the buttermilk with ¼ cup sugar. Warm it in the microwave in short bursts until it reaches 100-110°F. Sprinkle yeast over the surface and allow it to foam for about 5 minutes.

Pour the yeast mixture into a stand mixer bowl. Add the melted butter, egg, and egg yolk, and whisk to combine everything.

Gradually add in the flour, salt, and lemon zest. Using the dough hook, mix the ingredients until a sticky dough forms. Alternatively, knead by hand until the dough is smooth.

On a floured surface, transfer the dough and knead for a few minutes until elastic. Place it in a greased bowl, cover it, and let it rise in a warm spot for about 1 hour or until doubled in size.

While the dough rises, prepare the blueberry filling by heating the blueberries, lemon juice, ¼ cup sugar, and cornstarch in a saucepan over medium heat. Stir constantly until the berries break down and the mixture thickens, about 5 minutes. Set aside to cool.

Roll the dough out to a rectangular shape about 13 x 16 inches and ¼ inch thick. Spread the softened butter over the dough, followed by an even layer of the brown sugar and cinnamon mixture. Then spread the cooled blueberry mixture on top.

Gently roll the dough from the long side into a log shape, sealing the edges by pinching them together. Slice the log into 12 even rolls using a knife or dental floss.

Arrange the rolls cut-side up in a buttered 9 x 13-inch baking dish. Cover and let them rise again for 20 minutes. Preheat the oven to 375°F while the rolls rise.

Bake the rolls for 20-25 minutes, or until they are golden and puffed up in the center.

While the rolls bake, whisk together powdered sugar, lemon zest, and lemon juice to create a smooth glaze. Adjust consistency by adding more powdered sugar or lemon juice if needed.

Once out of the oven, generously drizzle the glaze over the warm rolls. Garnish with fresh blueberries if desired.