Butter bottom and sides of a 9x13-inch glass baking dish. Cut loaf into 1-inch slices. Butter one side of each slice, cut slices once on the diagonal, and arrange slices butter side up, slightly overlapping.

In a large mixing bowl, whisk together eggs, milk, sugar, vanilla, bourbon (optional), nutmeg, cardamom, and salt. Pour custard over bread. Cover with plastic wrap and chill in refrigerator overnight.

Remove pan from refrigerator and let it come to room temperature, about 30 minutes. Bring a kettle of water to boil over high heat.

Preheat oven to 325°. Set the baking pan into a roasting pan large enough to hold it and pour in enough boiling water to come halfway up the sides. Bake, uncovered, 50–60 minutes, until custard has set and toast is lightly browned. Serve warm with maple syrup.