1. Make the blueberry filling:

1. In a small saucepan, combine blueberries, sugar, lemon juice, and zest.

2. Cook over medium heat until berries start to burst (about 5 minutes).

3. Stir in the cornstarch slurry and cook 1–2 more minutes until thickened.

4. Remove from heat and let cool completely.

2. Activate the yeast:

1. In a bowl, combine warm milk, yeast, and 1 Tbsp of sugar.

2. Let sit for 5–10 minutes until foamy.

3. Make the dough:

1. In a large mixing bowl (or stand mixer with dough hook), whisk together remaining sugar, eggs, vanilla, lemon zest, and salt.

2. Add the yeast mixture and half the flour; mix until combined.

3. Add remaining flour and knead (5–8 minutes) until a soft dough forms.

4. Gradually add the softened butter, one tablespoon at a time, kneading until fully incorporated and smooth.

   • Dough should be silky and elastic, slightly tacky but not sticky.

5. Place in a greased bowl, cover, and let rise for 1 ½–2 hours, or until doubled.

4. Shape and fill:

1. Punch down the risen dough and roll it out into a rectangle (~12×16 inches).

2. Spread the cooled blueberry filling evenly over the surface.

3. Roll up tightly from the long side into a log.

4. Slice the log in half lengthwise (exposing the filling), then twist the two halves together into a braid.

5. Transfer to a greased loaf pan or shape into a freeform twist on a parchment-lined baking sheet.

6. Cover and let rise another 45–60 minutes, until puffy.

5. Bake:

• Preheat oven to 350°F (175°C).

• Bake for 30–35 minutes, or until golden brown and an instant-read thermometer reads 190°F (88°C) in the center.

• If the top browns too quickly, cover loosely with foil halfway through.

6. Glaze:

1. Whisk together powdered sugar, lemon juice, and zest until smooth.

2. Drizzle over the slightly cooled bread.

3. Slice and serve warm or at room temperature.