Step 1: Preheat your oven to 350°F (175°C). Lightly grease a 9x13 inch baking dish.
Step 2: In a large mixing bowl, whisk together the whole milk, heavy cream, eggs, granulated sugar, vanilla extract, ground cinnamon, ground nutmeg, and salt until well combined and the sugar has dissolved.
Step 3: Add the cubed stale brioche and challah bread to the custard mixture. Gently toss to ensure all bread cubes are coated. If using, stir in the optional raisins or dried cranberries.
Step 4: Let the bread soak in the custard for at least 30 minutes at room temperature, or cover and refrigerate for several hours or overnight for best results. This allows the bread to fully absorb the liquid, ensuring a custardy interior.
Step 5: Pour the bread and custard mixture into the prepared baking dish. Drizzle the melted butter evenly over the top.
Step 6: Bake for 45-55 minutes, or until the top is golden brown and a knife inserted into the center comes out mostly clean (a little custardy liquid is fine). The pudding should have a slight wobble in the center.
Step 7: Remove from the oven and let it cool for 10-15 minutes before serving. This allows the pudding to set further.
Step 8: Dust with powdered sugar, if desired, and serve warm with your favorite toppings like vanilla ice cream, whipped cream, or caramel sauce.