Start by preheating your oven to 350 degrees and greasing a 1 1/2 to 2 quart baking dish.

Cut down your conchas into 1-inch cubes, and place in your pan.

In a large bowl, whisk together the milk, condensed milk, eggs, vanilla, and pumpkin pie spice together until fully mixed. Add in your concha cubes, and give them a good toss, making sure every piece is coated in your milk mixture.

Pour your milk and bread mixture into your greased pan, and let it sit for about 5 minutes. Sprinkle the chopped walnuts and butter over the bread, loosely cover the pan with aluminum foil and bake for 50 minutes to an hour until the top is golden brown and a toothpick inserted into the custard comes out clean.

After its done let it cool for 30 minutes, dust with powdered sugar, and serve.