Spread one side of each bread slice with butter and arrange, butter side up, in an ungreased, 13×9-inch pan. (If desired, you may first cut the bread slices in half on the diagonal. Depending on the exact number and shape of slices, you may need to put a few of the slices in a smaller second pan so they fit in one layer.)
In a medium-size bowl, whisk together the remaining ingredients and pour over the bread.
Cover the pan, and refrigerate for at least four hours or overnight.
When ready to bake, preheat the oven to 350 degrees F. Remove the bread from the pan, allowing any excess egg mixture to drip off, and place on a well-greased baking sheet.
Bake for 25-30 minutes, flipping after 15-20 minutes, or until golden and slightly crusty. Depending on your oven and the thickness of the bread slices, a few extra minutes may be necessary to cook through middle.
Serve with powdered sugar or maple syrup, as desired.