Grease an 8x11-inch ceramic dish (I use an oval one, but any shape is good) with 1 tablespoon unsalted butter.
Arrange 1 pound of bread cubes or chunks in the dish, sprinkling ½ cup raisins in between.
In a medium mixing bowl, stir 3 eggs, 1 ¾ cups milk, ¼ cup cream, ⅓ cup sugar, ¼ teaspoon cinnamon, ½ teaspoon vanilla and 1 teaspoon zest. Integrate well without beating, as we don't want to incorporate air, but make sure the eggs are well-mixed.
Pour the egg mixture carefully over the bread, making sure all is moistened. Since the bread is usually uneven, some slices might be more covered than others, which is fine.
Let this mixture stand at room temperature, allowing the bread to absorb the liquid for 20-30 minutes. You might want to leave it longer if it is dry. You can also cover the dish with plastic wrap and refrigerate it for a few hours.
Preheat the oven to 375°F (190°C).
Check the mixture once or twice, submerging again any bread slices that popped to the surface too much. If they're on the surface and one of the sides is not being soaked enough, turn them over.
Mix 1 tablespoon of sugar with ¼ teaspoon ground cinnamon and sprinkle on top.
Bake for 20 minutes, decrease the temperature to 325°F (160°C) and bake for another 20-25 minutes until the top is puffed and golden brown. A tester or toothpick inserted should come out clean. Don't be tempted to leave it until it's very firm because we want it to be creamy. But also make sure the custard is not too runny.
Let it cool a little, then serve warm with whipped cream or ice cream if you like. Drizzle some caramel or dulce de leche on top for an even more decadent dessert.
Store leftovers, covered, in the refrigerator.