In a large, 10-inch ovenproof skillet, preferably cast iron, on low heat melt the butter and swirl so it coats the bottom and the sides. (Alternatively, you can add melted butter to a 9×9 square pan.)
Preheat the oven to 400° F. Let the casserole sit unrefrigerated for 20 minutes, until some of the milk mixture has been absorbed, and then bake covered for 25 minutes. Remove the cover and then bake uncovered for 20 minutes or until top is firm and lightly browned. Let the casserole rest for 10 minutes before serving. Sprinkle with powdered sugar and serve with syrup and/or sour cream.
You could also make this with thick Texas toast bread or even leftover buns, if you like. And if you wanted to add nuts, that would be tasty, too.
French toast casserole with blueberries and sausage | www.homesicktexan.com