Step 1

Arrange a rack in the middle of the oven and heat the oven to 300℉. Meanwhile, cut 1 loaf unsliced challah or brioche bread into 1-inch cubes. Place in an even layer on a rimmed baking sheet.

Step 2

Bake until the bread is dry but not browned, about 20 minutes. Set aside until mostly cool.

Step 3

Meanwhile, increase the oven temperature to 350℉. Lightly coat a 9x13-inch baking dish with unsalted butter. Melt 3 tablespoons unsalted butter in a small microwave-safe bowl in 20-second bursts until mostly melted, 30 to 40 seconds total. (Alternatively, melt the butter in a small saucepan over medium heat.) Set aside to cool slightly.

Step 4

Place 4 large eggs and 2 large egg yolks in a large bowl and whisk to break up. Finely grate the zest of 1 medium orange or lemon into the bowl if desired; reserve the fruit for another use. Add 4 cups whole milk or half-and-half, 1 cup granulated sugar, 2 tablespoons vanilla extract, 1 teaspoon ground cinnamon, 1/2 teaspoon ground nutmeg, and 1/4 teaspoon kosher salt. Whisk until combined.

Step 5

Add the bread and 1/2 cup dried fruit or chocolate chips if using. Gently stir to coat the bread and press the bread down slightly to soak up the custard.

Step 6

Transfer to the baking dish and arrange into an even layer. Pour any remaining liquid over the top. Drizzle the melted butter over the top.

Step 7

Bake until puffed, golden brown, the bread pulls away from the edges of the pan, and the custard is set, 45 to 50 minutes. Let cool for 5 minutes before serving. Serve topped with powdered sugar or vanilla ice cream if desired.