Preheat oven to 350°F.
Grease a 9x13-inch or 3-quart baking dish with ½ tbsp butter. Arrange sliced bread evenly in the dish.
In a large bowl, whisk together eggnog, milk, eggs, sugar, Cointreau, vanilla extract, and salt.
Pour custard mixture over the bread. Press bread gently to help it absorb. Cover with foil and refrigerate for 1 hour (or overnight).
Uncover and bake for 45–50 minutes (55 minutes if using glass), until custard is set and the top is lightly golden.
Let cool for 10 minutes. Dust with powdered sugar, top with berries, and drizzle with maple syrup before serving.