01 - Preheat oven to 350°F. Grease an 8x8-inch baking dish with butter.
02 - Place cinnamon roll bread cubes evenly in the dish, layering raisins or sultanas between slices if using.
03 - Whisk eggs, granulated sugar, vanilla, cinnamon, nutmeg, and salt in a large bowl until blended.
04 - Add whole milk and heavy cream to the egg mixture, whisking until smooth.
05 - Pour custard evenly over bread cubes, pressing gently to saturate all pieces. Let rest for 10 minutes to absorb.
06 - Dot surface with softened butter and sprinkle demerara sugar for added texture, if desired.
07 - Bake in preheated oven for 35 to 40 minutes, until custard is set and top turns golden brown.
08 - Remove from oven, allow to cool slightly, then dust with powdered sugar before serving warm.