This recipe is best enjoyed right away, but you can make it ahead of time if you really need to. Just mix everything as directed without the extra brown butter drizzle at the end. Store the mash and the brown butter separately in the fridge for up to 3 days. When you're ready to serve, reheat the mash (I'd just microwave it, but you can bake it if you really want to!) then melt and drizzle the brown butter on top to serve.