Layer half of the croissant pieces in a 9x13 baking dish.
In a large pan over medium/low, melt the butter, stir in the brown sugar then add 1/4 cup heavy cream. Stir until fully combined and smooth. Add the raisins and heat 1-2 more minutes or until raisins plump. Remove from heat.
Combine 1/2 cup heavy cream, buttermilk, rum extract, eggs and salt. Stir until smooth and pale yellow.
Pour half of the custard mixture over the croissants in baking dish. Then pour half of caramel/raisin mixture. Top with remaining croissant pieces. Pour remaining custard and caramel on top. Gently press down with the back of a spoon. Refrigerate 1-2 hours.
Preheat oven to 375°F
Bake 20-25 minutes or until lightly browned on top. remove and let cool 10 minutes.