Preheat the oven to 375°F. Generously butter a 6 to 8 cup baking dish (we used an 11 x 7 inch casserole dish, but you can use whatever you have on hand). Set aside.
In a large mixing bowl combine the sugar, milk, rum, eggs, cinnamon, and salt. Use a whisk to thoroughly mix until completely homogenous.
Add the croissant pieces to the mixture and toss to evenly coat. Last, fold in the raisins. Pour the mixture into the prepared pan.
Bake for 30 to 40 minutes or until the bread pudding has puffed up and is golden brown along the edges.
While the bread pudding cools, make the creme anglaise. In a medium sauce pan over medium heat, add the cream, sugar, rum and butter. Simmer until the mixture starts to bubble around the edges. Make sure to keep an eye on it–you don’t want it to boil or burn. Scoop out a bit of the mixture and mix it with the flour to create a thin paste. Slowly add the flour paste into the simmering sauce, whisking to combine. Allow it simmer for about 2 minutes or until the sauce has reduced and coats the back of a spoon.
Take off the heat and allow to cool slightly before drizzling over the warm bread pudding. Enjoy warm!