Generously butter a 9×13-inch baking dish, ensuring every corner is covered.

Layer half of the bread cubes in the dish, followed by ⅔ of the blueberries. Add remaining bread and top with the rest of the blueberries.

In a mixing bowl, whisk together eggs, milk, heavy cream, vanilla extract, 6 tablespoons sugar, lemon zest, and kosher salt until frothy.

Pour the custard mixture over the layered bread and blueberries, pressing down gently to soak.

Allow to soak for at least 30 minutes at room temperature or refrigerate overnight for best flavor.

Preheat oven to 350°F (175°C).

Sprinkle the remaining 2 tablespoons of sugar on top and dot with diced unsalted butter.

Bake for approximately 45 minutes until golden brown. Let it cool for 10 minutes before serving.