Start by preparing the tangzhong. In a small saucepan, whisk together the milk and flour. Cook over medium-low heat, stirring constantly, until it thickens to a pudding-like consistency, about 2-3 minutes. Use a thermometer to ensure it reaches 148-155°F. Once done, transfer it to a bowl to cool to room temperature.

In a large bowl or stand mixer, combine the tangzhong, sourdough discard, lemon zest, egg, egg yolk, whole milk, granulated sugar, instant yeast, and vanilla extract. Whisk until the mixture is smooth.

Add the flour and salt to the wet ingredients. Knead the mixture in a stand mixer on Speed 1 or by hand for about 3-4 minutes until the dough comes together.

If substituting the sourdough discard with extra milk and flour, add the milk with the wet ingredients and the flour with the dry ingredients.

Once the dough is formed, gradually incorporate the softened butter, adding one tablespoon at a time until fully mixed in. The dough should be smooth and slightly sticky.

Grease a large mixing bowl and place the dough inside, covering it with a lint-free kitchen towel. Let it rest for 20 minutes.

After resting, knead the dough by hand for a minute or two to soften it further and develop the gluten. Cover the bowl again and let the dough rise at room temperature for about 1 ½ hours, until it is about 75-90% risen.

While the dough is rising, prepare the blueberry filling. In a small saucepan, combine the blueberries, sugar, lemon juice, lemon zest, and salt. Cook over medium heat, stirring often, until the mixture simmers and the berries begin to break down, about 10 minutes.

Mix the cornstarch and water in a small bowl to create a slurry, then stir it into the blueberry mixture. Cook for an additional minute until the mixture thickens and becomes glossy. Remove from heat and let it cool completely, covering it with plastic wrap to prevent a skin from forming.

Lightly grease a 9x5-inch loaf pan and line it with parchment paper, leaving some overhang for easy removal.

On a floured surface, roll the dough into a 12x16-inch rectangle. Spread softened butter evenly over the dough, leaving a ¼-inch border around the edges. Then, spread the cooled blueberry compote over the butter layer, again leaving a border.

Using a pizza cutter or sharp knife, cut the dough lengthwise into four 3-inch-wide strips, then cut each strip crosswise every 4 inches to create 16 rectangles.

Fold each rectangle in half with the filling inside, creating a 3x2-inch piece. Layer these folded pieces upright in the prepared pan until all are snugly fit.

Cover the pan loosely with plastic wrap and let it rise for 45 minutes to an hour, until the dough is puffy and the top layers rise above the rim of the pan.

Preheat the oven to 350°F (175°C) and place a baking sheet on the rack below to catch any drips.

Bake the bread for about 55-60 minutes, tenting with foil during the last 20 minutes to prevent over-browning. The bread is done when it feels firm to the touch and the center registers 190-195°F.

Allow the bread to cool while you prepare the lemon cream cheese frosting. In a bowl, beat the cream cheese and softened butter until smooth. Add the vanilla, powdered sugar, lemon juice, lemon zest, and salt, mixing until combined.

Gradually add milk, one tablespoon at a time, until the frosting reaches a drizzleable consistency. If too thick, warm it in the microwave for 15 seconds and mix again.

Remove the bread from the pan and drizzle the frosting over the top. Optionally, garnish with additional lemon zest. Serve and enjoy!