Poffins are an essential part of a Pokémon’s diet to increase their Sheen. Before you can get to baking these lovely treats, you first need to get your hands on a Poffin Case. This key item is hidden somewhere in the game, and it is up to you to find it if you want to help your Pokémon shine at the contest hall. Here is where you can find the Poffin Case in Pokémon Brilliant Diamond and Shining Pearl.
Before you can get the Poffin Case, you first need to progress through the game far enough to reach Hearthome City. You will reach this area after defeating the gym leaders in Oreburgh City and Eterna City. Once you reach Hearthome City, head over to the Pokémon Fan Club building. You can find the building by heading east of the PokeMart. The building you are looking for is blue with an orange roof.
Go inside the building and talk to the old man behind the table. After a little rant, he will give you the Poffin Case. If you want to start creating some Poffins, head out of the house and go west. To the left of the PokeMart is another small house with a green roof. You can start cooking here.