Step 1

Butter a shallow 2-quart baking dish. In a large bowl, whisk together eggs milk, sugar, cinnamon, and salt. Dip brioche in egg mixture, then arrange in dish, overlapping slightly. Pour remaining egg mixture over brioche, cover with plastic wrap, and let stand 30 minutes or refrigerate up to overnight.

Step 2

Preheat oven to 325 degrees. Sprinkle blueberries in between and on top of the bread slices. Bake until puffed and custard is just set, 55 to 65 minutes (tent with foil if browning too quickly). Let cool 15 minutes. Dust with confectioners' sugar just before serving.