Preheat oven to 350°F (175°C). Grease a 9x13-inch baking dish with butter or nonstick spray.

Cut the sourdough into roughly 1-inch cubes and spread them evenly in the prepared dish.

In a medium bowl, whisk the eggs, milk, 1 teaspoon cinnamon, sugar, and vanilla until smooth.

Pour the custard evenly over the bread. Press the bread gently so the cubes begin to absorb the liquid.

Cover the dish with plastic wrap or a lid and refrigerate overnight so the bread soaks up the custard.

Next morning, uncover and sprinkle the top with the remaining cinnamon and a little extra sugar. Dot with the small pieces of butter.

Bake for 30 to 40 minutes, until the top is golden and the center is set but still slightly custardy. A toothpick inserted in the center should come out mostly clean.

Let rest for 5 minutes, then cut into squares and serve warm with a generous drizzle of maple syrup.