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