Add the butter to a microwave safe dish and microwave until it melts. Coat a large baking dish or large cast iron skillet with the melted butter.

In a large mixing bowl, whisk together the milk, buttermilk, eggs, sugar, and vanilla.

Slice the bread on a diagonal. Gently press the slices and arrange them in the dish with the corner facing up. Pour the egg mixture over the slices of bread. Using your hands, gently press the bread down into the liquid so it soaks up as much as possible. Cover with plastic wrap and refrigerate for at least thirty minutes and up to overnight.

Preheat the oven to 350 degrees F.

In a small bowl combine the apples, walnuts, sugars, cinnamon, salt, maple syrup, lemon juice. Mix well to coat all the apples.

Remove the soaked bread from the refrigerator and remove the plastic wrap. Pour the apple mixture on top of all the bread. Evenly spread it out using a rubber spatula. Bake in the oven for 35 – 40 minutes until golden brown.

Let cool for a few minutes before serving and then lightly dust with powdered sugar.