Begin by preparing the levain. In a small bowl, mix together the sugar, active sourdough starter, whole milk, and all-purpose flour. Allow it to sit until bubbly and doubled in size.

For the tangzhong, combine the whole milk and all-purpose flour in a small saucepan over low heat. Stir continuously until the mixture thickens and resembles a paste. Remove from heat and let it cool slightly.

In a large mixing bowl, combine the all-purpose or bread flour, sugar, eggs, the prepared levain, cooled tangzhong, whole milk, and sea salt. Mix until a cohesive dough forms.

Knead the dough until it becomes smooth and elastic, about 10 minutes by hand or 5 minutes in a stand mixer.

Incorporate the softened butter gradually into the dough until fully absorbed. The dough should become glossy and elastic.

Allow the dough to rise in a greased bowl, covered with a kitchen towel until doubled in size, about 1 to 2 hours.

While the dough is rising, prepare the filling. In a bowl, combine the frozen blueberries, sugar, lemon juice, and lemon zest. Stir gently.

Once the dough has risen, punch it down and roll it out into a rectangle on a lightly floured surface, about ¼ inch thick.

Spread the blueberry filling evenly over the rolled-out dough, leaving a small border.

Roll the dough tightly from one end to the other, creating a log shape and pinch the edges to seal.

Slice the dough log in half lengthwise, twist the halves together, and place into a greased baking dish.

Cover loosely with a kitchen towel and let it rise again until puffy, about 30 minutes.

While the babka rises, combine the sugar, water, and lemon juice in a small saucepan. Simmer until the sugar dissolves.

Preheat the oven to 350°F (175°C). Once risen, bake the babka for about 30-35 minutes until golden brown.

After removing from the oven, brush the warm syrup over the babka.