Arrange the brioche slices into a buttered 9 inch square pan.
In a saucepan, heat the milk, butter and orange zest on a low flame, just until the butter melts. Turn the heat off and add in your mascarpone. Stirring until it melts down.
Meanwhile, in another bowl add your sugar, eggs, vanilla, rum and orange juice. Whisk until thoroughly incorporated. Slowly add the cooled milk mixture into the egg mixture. Once that’s well combined, add in the salt and stir.
Pour the custard over the bread and allow it to sit for 30-45 minutes. Meanwhile preheat your oven to 350.
Bake for 30 minutes and then remove it from the oven and sprinkle sugar evenly on top. Turn your oven on to broil and pop it back in for another 2-3 minutes. Make sure to keep a close eye on it as it can burn easily. You just want to melt the sugar down. It is best served warm. Serve this with ice cream or dessert sauce of your choice. Enjoy!