Grease a 13x9-inch baking dish with butter or nonstick spray.
Spread the prepared bread cubes and apple pieces in the prepared baking dish.
For the custard, in a large mixing bowl, whisk together eggs, evaporated milk, half of the spiced apple cider drink mix, 1/4 cup brown sugar, and vanilla extract until well combined.
Pour the custard evenly over the top of the bread and apples. Gently tap to ensure all of the bread cubes are submerged.
If desired, sprinkle chopped pecans or walnuts on top of the bread. Cover with plastic wrap or aluminum foil and refrigerate for at least 1 hour or overnight.
Once ready to bake, remove the casserole from the refrigerator and let it sit at room temperature for about 15 minutes. Preheat oven to 375°F.
In a small bowl, whisk together butter and the remaining half of the spiced apple cider drink mix and 1/4 cup brown sugar. Pour the mixture over top of the prepared base.
Bake uncovered for 35-40 minutes, or until the top is golden brown and the center is set.
Let the baked French toast cool for a few minutes. Dust with powdered sugar if desired and serve with your favorite toppings.