Croissant French Toast Bake with Berries

Name: Croissant French Toast Bake with Berries
Ingredients: 8 large croissants (, cut in half lengthwise); 2 cups half and half or whole milk; 1 teaspoon pure vanilla extract; 6 large eggs; 1/2 cup light brown sugar (, lightly packed); 1/2 teaspoon ground cinnamon; 1/4 teaspoon ground nutmeg; 1/4 teaspoon kosher salt; 1 cup strawberries (, cut into 3/4- to 1-inch pieces); 1 cup blueberries; powdered sugar (, for dusting baked casserole); lightly-sweetened whipped cream (, for serving (optional)); 1 cup pure maple syrup; 1 tablespoon dark rum