Heat oven to 350°F. Grease a 2-quart baking dish with a little butter.
Toss the bread cubes with the softened butter until lightly coated. Stir in the raisins.
Whisk eggs, sugar, milk, cream, vanilla, cinnamon, nutmeg, and salt until smooth.
Fill the dish with the buttered bread mixture. Pour the custard evenly over the top.
Press the bread down to submerge. Let it soak for 10 minutes.
Sprinkle with coarse sugar. Set the dish on a baking sheet to catch drips.
Bake until puffed, golden, and just set in the center, 35–40 minutes.
Cool 10 minutes. Serve warm, plain or with cream or ice cream.